Applications of ISAT, Interchange Safety Analysis Tool (ISAT): User Manual
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Default Data
This section presents the default data that are provided with ISAT. These data include:
- Safety performance functions (SPFs).
- Calibration coefficients.
- Crash distributions by severity and type.
In the case of default SPFs, it is anticipated that most ISAT users will not modify the default coefficients of the SPFs. In other words, it is expected that in most instances ISAT will be run utilizing the default SPFs. In this respect, users will very seldom change the values associated with the default SPFs. Concerning the default calibration coefficients and crash distributions by severity and type, the user should modify these default values to more accurately reflect the safety experience of their local/State agency prior to using ISAT to perform actual safety assessments. Then, it is recommended that the user modify these default values on an annual basis. The user can modify the default data by clicking on the hyperlinks located on the Input-General worksheet. Five hyperlinks are provided to access the default SPFs, one hyperlink is provided to access the default calibration coefficients, and four hyperlinks are provided to access the default crash distributions. The following sections present more detailed information on each type of default data provided within ISAT and further explain the need to modify these default data.
Safety Performance Functions (SPFs)
For many years, statistical techniques have been applied to predict the crash experience along roadway segments and at intersections. These statistical models are developed using crash and inventory databases, selecting an appropriate functional form for the model, and using regression techniques to estimate the values of the coefficients and parameters in the model. Historically, these models were developed with multiple regression techniques. More recently, Poisson and negative binomial (NB) analyses have been used because theoretically they are better suited to crash data with low-frequency observations. These statistical models are often referred to as safety performance functions (SPFs).
ISAT makes use of SPFs for predicting and/or estimating crash frequencies for individual components of an interchange and the surrounding roadway network. Safety estimates are calculated for the individual components, and these safety estimates are summed to obtain safety performance estimates for the interchange as a whole. Within ISAT default SPFs are provided for the following primary interchange elements that can be included in an analysis area:
- Mainline freeway segments.
- Interchange ramps (and acceleration lanes).
- Crossroad ramp terminals and intersections.
- Crossroad roadway segments.
ISAT was developed using existing knowledge of SPFs or knowledge acquired from other ongoing safety research projects. During the development of ISAT, a review of the technical literature on safety assessment of freeway interchanges was conducted. This review focused on quantitative information that was potentially applicable for use in ISAT. Specifically, interchange-safety-related literature was reviewed to identify quantitative SPFs which could potentially be incorporated into ISAT. The review included both engineering and statistical considerations. The SPFs selected for incorporation into ISAT were obtained from the following primary sources:
Development of Safety Performance Functions for SafetyAnalyst Interim Tools4
SafetyAnalyst is a set of computerized analytical tools being developed for FHWA to aid State and local highway agencies in highway safety management. The main purpose of SafetyAnalyst is to improve a highway agency's systemwide programming of site-specific safety improvements. SafetyAnalyst incorporates state-of-the-art safety management approaches for guiding the decision-making process to identify safety improvement needs and has a strong basis in cost-effectiveness analysis. SafetyAnalyst will help highway agencies get the greatest possible safety benefit from each dollar spent in the name of safety.
Statistical Models for Interchange Ramps and Speed-Change Lanes3
The objective of this research was to develop statistical models for defining the relationship between traffic crashes and highway geometric design elements and traffic volumes for interchange ramps and speed-change lanes. The models were developed using Highway Safety Information System (HSIS) data for interchange ramps and speed-change lanes in the State of Washington. Additional geometric design features were obtained from a review of interchange diagrams, aerial photographs, and other existing highway agency files.
It is important to recognize that the accuracy of ISAT is only as good as the safety knowledge on which it is based. The default SPFs provided within ISAT were selected for incorporation in the tool based primarily for the following reasons: (1) statistical validity of the models, (2) criticality of the components based upon engineering judgment, and (3) methodological consistency.
Due to the comprehensive process for selecting SPFs for incorporation within ISAT, it is anticipated that most users will run the program utilizing the default SPFs; however, the program does not prohibit users from modifying the coefficients of the default SPFs. If users have models that better reflect local safety experience, users can replace the default coefficients with new values. This should only be done if the models were developed with the same functional form as the default SPFs. The following sections provide information on the sources of the default SPFs, their functional form, and select coefficients and parameters of the models.
Mainline Freeway Segment SPFs
The default SPFs for mainline freeway segments provided within ISAT are located on the SPFs Mainline Freeways worksheet and are accessible only through the Go To SPFs for Mainline Freeways hyperlink on the Input-General worksheet. The default SPFs for mainline freeway segments were developed for use within SafetyAnalyst. These SPFs predict the number of crashes that may occur on mainline freeway segments. SPFs are provided for two unique types of mainline freeway segments:
- Mainline freeway segments within an interchange area.
- Mainline freeway segments outside an interchange area.
As described in the Mainline Freeway Segment Inputs section, the SPFs for mainline freeway segments within interchange areas attempt to account for the increased level of weaving, lane changing, and acceleration/deceleration that takes place immediately upstream, downstream, and between interchange ramps. The SPFs for mainline freeway segments outside an interchange area model the safety experience of basic mainline freeway segments having homogenous characteristics. In these homogeneous segments, the number of through lanes remains the same, the traffic volume remains the same throughout the segment because there are no ramps associated with the segment, and the primary movements include lane changes between through lanes.
The limits of mainline freeway segments within interchange areas are defined in general terms to extend approximately 0.48 km (0.3 mi) upstream from the gore of the first ramp of a particular interchange to approximately 0.48 km (0.3 mi) downstream from the gore of the last ramp of the given interchange. All mainline freeway segments beyond these boundaries are by definition mainline freeway segments outside an interchange area.
The SPFs for mainline freeway segments within interchange areas include crashes that occur on the speed-change lanes (i.e., deceleration and acceleration lanes) adjacent to the mainline freeway segment through lanes, but the SPFs do not predict crashes that occur on the ramp proper (i.e., downstream from the gore point for off-ramps and upstream from the gore point for on-ramps). Because the SPFs for mainline freeway segments within interchange areas include crashes that occur on speed-change lanes, certain methodological assumptions are made within ISAT so that crashes are not double counted on mainline freeway segments and acceleration lanes.
Table 15 presents the coefficients and parameters of the default SPFs for mainline freeway segments provided within ISAT. Table 15 also shows the functional form of the models. In general, all models included in ISAT were developed using NB regression. Table 16 defines the coefficients and parameters of the defaults SPFs. Twenty SPFs for mainline freeway segments are provided within ISAT and are a function of the following:
- Area type.
- Interchange area (i.e., within or outside interchange area).
- Number of lanes.
- Severity level.
Portions of the SPFs Mainline Freeways worksheet are protected. In other words, if the user tries to modify the value of a protected cell, a message will appear indicating the cell is protected, and the user is restricted from modifying the default value in the given cell. On the SPFs Mainline Freeways worksheet, the cells with column headings for SPF No., Area type, Interchange area, Number of through lanes, and Severity level are protected. The user may only modify the default values for the Log intercept (a), Coeff of log ADT (b), dispersion parameter, and Max ADT.
Ramp SPFs
Two sets of default SPFs associated with ramps are provided within ISAT. The default SPFs used to predict the safety performance for the ramp proper are located on the SPFs Ramp worksheet and are accessible only through the Go To SPFs for Ramps hyperlink on the Input-General worksheet. The default SPFs used to predict the safety performance for acceleration lanes are located on the SPFs Accel Lanes worksheet and are accessible only through the Go To SPFs for Accel Lanes hyperlink provided on the Input-General worksheet. The default SPFs for the ramp proper were developed for use within SafetyAnalyst. The default SPFs for acceleration lanes were developed by Bauer and Harwood.3
Table 17 presents the coefficients and parameters of the default SPFs for ramps (i.e., ramp proper) provided within ISAT. Table 17 also shows the functional form of the models. Table 18 defines the coefficients and parameters of the defaults SPFs. Twenty-eight SPFs for ramps (i.e., ramp proper) are provided within ISAT and are a function of the following:
- Area type.
- Ramp type.
- Ramp configuration.
- Severity level.
The ramp SPFs within SafetyAnalyst were developed by combining the data for rural and urban areas; thus, separate ramp SPFs are not necessarily available for rural and urban areas. For consistency purposes the SPFs are presented by area type, but the coefficients and parameters are the same for both area types.
Portions of the SPFs Ramps worksheet are protected. The user may only modify the default values for the Log intercept (a), Coeff of ADTramp (b), Coeff of ramp length (e), dispersion parameter, and Max ADT.
Table 15. SPF Coefficients and parameters for mainline freeway segments.
SPF No. |
Area Type |
Interchange Area |
Number of Through Lanes (Directional) |
Severity Level |
Log Intercept (a) |
Coeff of Log ADT(b) |
Dispersion Parameter |
Max ADT |
numeric |
(U, R) |
(Y, N) |
(2, 3, 4) |
(TOT, FI) |
numeric |
numeric |
numeric |
(veh/day) |
SafetyAnalyst Roadway Segment SPF |
N = ea x ADTb x SL |
DMF1 |
DMF2 |
DMF3 |
DMF4 |
DMF5 |
DMF6 |
DMF7 |
DMF8 |
DMF9 |
1 |
R |
Y |
2 |
TOT |
-7.28 |
0.92 |
0.45 |
60,621 |
2 |
R |
Y |
3 |
TOT |
-10.05 |
1.14 |
0.42 |
197,798 |
3 |
U |
Y |
2 |
TOT |
-11.23 |
1.30 |
0.81 |
241,255 |
4 |
U |
Y |
3 |
TOT |
-11.25 |
1.28 |
0.60 |
255,154 |
5 |
U |
Y |
4 |
TOT |
-26.76 |
2.58 |
0.52 |
233,323 |
6 |
R |
Y |
2 |
FI |
-8.68 |
0.94 |
0.58 |
60,621 |
7 |
R |
Y |
3 |
FI |
-12.07 |
1.22 |
0.39 |
197,798 |
8 |
U |
Y |
2 |
FI |
-12.89 |
1.38 |
0.79 |
241,255 |
9 |
U |
Y |
3 |
FI |
-13.62 |
1.42 |
0.55 |
255,154 |
10 |
U |
Y |
4 |
FI |
-25.63 |
2.42 |
0.53 |
233,323 |
11 |
R |
N |
2 |
TOT |
-6.46 |
0.79 |
0.17 |
60,621 |
12 |
R |
N |
3 |
TOT |
-9.67 |
1.07 |
0.24 |
190,403 |
13 |
U |
N |
2 |
TOT |
-7.85 |
1.00 |
0.99 |
151,038 |
14 |
U |
N |
3 |
TOT |
-5.96 |
0.78 |
0.48 |
241,255 |
15 |
U |
N |
4 |
TOT |
-16.24 |
1.67 |
0.45 |
223,088 |
16 |
R |
N |
2 |
FI |
-8.86 |
0.9 |
0.10 |
60,621 |
17 |
R |
N |
3 |
FI |
-11.67 |
1.17 |
0.21 |
190,403 |
18 |
U |
N |
2 |
FI |
-8.82 |
1.02 |
1.15 |
151,038 |
19 |
U |
N |
3 |
FI |
-7.60 |
0.85 |
0.54 |
241,255 |
20 |
U |
N |
4 |
FI |
-19.16 |
1.85 |
0.52 |
223,088 |
Table 16. Definitions of coefficients and parameters of SPFs for mainline freeway segments.
VariableName |
Variable No. |
Format |
Definition |
SPF No. |
DMF1 |
Numeric |
Each SPF is identified by a sequential integer, starting with 1; this variable is not used in calculations but is useful for organization. |
Area type |
DMF2 |
Character(U, R) |
General character of land use surrounding the interchange, preferably based on FHWA urban area boundaries:
U = Urban
R = Rural |
Interchange area |
DMF3 |
Character(Y, N) |
A code identifying whether the segment is located within the interchange area:
Y = yes, segment is located within the interchange area
N = no, segment is not located within the interchange area |
Number of through lanes (directional) |
DMF4 |
Numeric(2, 3, 4) |
This field includes all lanes on the segment in a given direction that are used by through traffic; it does not include auxiliary lanes. |
Severity level |
DMF5 |
Character(TOT, FI) |
A code identifying the crash severity level:
TOT = total crashes
FI = fatal and injury crashes |
Log intercept (a) |
DMF6 |
Numeric |
Intercept of SPF. |
Coefficient of log ADT (b) |
DMF7 |
Numeric |
Coefficient of log ADT parameter (this ADT is bi-directional). |
Dispersion parameter |
DMF8 |
Numeric |
Dispersion parameter associated with negative binomial regression. |
Max ADT |
DMF9 |
Numeric(veh/day) |
The maximum traffic volume for which the coefficients of the SPF were calibrated/calculated. |
Table 17. SPF Coefficients and parameters for ramps.
SPF No. |
Area Type |
Ramp Type |
Ramp Configuration |
Severity Level |
Log Intercept (a) |
Coeff of ADTRamp (b) |
Coeff of Ramp Length (e) |
Dispersion Parameter |
Max ADT |
numeric |
(R, U) |
(ON, OFF, FWY) |
(D, PL, FFL, DIR) |
(TOT, FI) |
numeric |
numeric |
numeric |
numeric |
(veh/day) |
SafetyAnalyst Ramp Proper SPF |
N = ea x ADTRampb x RLe |
DR1 |
DR2 |
DR3 |
DR4 |
DR5 |
DR6 |
DR7 |
DR8 |
DR9 |
DR10 |
1 |
R |
OFF |
D |
TOT |
-3.17 |
0.45 |
1.0 |
1.49 |
22,566 |
2 |
R |
ON |
D |
TOT |
-8.28 |
1.03 |
1.0 |
2.57 |
24,966 |
3 |
R |
OFF |
PL |
TOT |
-4.50 |
0.73 |
1.0 |
1.17 |
22,538 |
4 |
R |
ON |
PL |
TOT |
-2.11 |
0.43 |
1.0 |
1.77 |
20,403 |
5 |
R |
OFF |
FFL |
TOT |
-4.50 |
0.73 |
1.0 |
1.17 |
22,538 |
6 |
R |
ON |
FFL |
TOT |
-2.11 |
0.43 |
1.0 |
1.77 |
20,403 |
7 |
R |
FWY |
DIR |
TOT |
-1.80 |
0.45 |
1.0 |
1.67 |
37,474 |
8 |
U |
OFF |
D |
TOT |
-3.17 |
0.45 |
1.0 |
1.49 |
22,566 |
9 |
U |
ON |
D |
TOT |
-8.28 |
1.03 |
1.0 |
2.57 |
24,966 |
10 |
U |
OFF |
PL |
TOT |
-4.50 |
0.73 |
1.0 |
1.17 |
22,538 |
11 |
U |
ON |
PL |
TOT |
-2.11 |
0.43 |
1.0 |
1.77 |
20,403 |
12 |
U |
OFF |
FFL |
TOT |
-4.50 |
0.73 |
1.0 |
1.17 |
22,538 |
13 |
U |
ON |
FFL |
TOT |
-2.11 |
0.43 |
1.0 |
1.77 |
20,403 |
14 |
U |
FWY |
DIR |
TOT |
-1.80 |
0.45 |
1.0 |
1.67 |
37,474 |
15 |
R |
OFF |
D |
FI |
-6.88 |
0.78 |
1.0 |
2.21 |
22,566 |
16 |
R |
ON |
D |
FI |
-14.40 |
1.61 |
1.0 |
3.44 |
24,966 |
17 |
R |
OFF |
PL |
FI |
-3.63 |
0.53 |
1.0 |
1.71 |
22,538 |
18 |
R |
ON |
PL |
FI |
-3.37 |
0.44 |
1.0 |
0.82 |
20,403 |
19 |
R |
OFF |
FFL |
FI |
-3.63 |
0.53 |
1.0 |
1.71 |
22,538 |
20 |
R |
ON |
FFL |
FI |
-3.37 |
0.44 |
1.0 |
0.82 |
20,403 |
21 |
R |
FWY |
DIR |
FI |
-2.80 |
0.46 |
1.0 |
1.89 |
37,474 |
22 |
U |
OFF |
D |
FI |
-6.88 |
0.78 |
1.0 |
2.21 |
22,566 |
23 |
U |
ON |
D |
FI |
-14.40 |
1.61 |
1.0 |
3.44 |
24,966 |
24 |
U |
OFF |
PL |
FI |
-3.63 |
0.53 |
1.0 |
1.71 |
22,538 |
25 |
U |
ON |
PL |
FI |
-3.37 |
0.44 |
1.0 |
0.82 |
20,403 |
26 |
U |
OFF |
FFL |
FI |
-3.63 |
0.53 |
1.0 |
1.71 |
22,538 |
27 |
U |
ON |
FFL |
FI |
-3.37 |
0.44 |
1.0 |
0.82 |
20,403 |
28 |
U |
FWY |
DIR |
FI |
-2.80 |
0.46 |
1.0 |
1.89 |
37,474 |
Table 18. Definitions of coefficients and parameters of SPFs for ramps.
Variable Name |
Variable No. |
Format |
Definition |
SPF No. |
DR1 |
Numeric |
Each SPF included in the analysis must be identified by a sequential integer, starting with 1; this variable is not used in calculations but is useful for organization. |
Area type |
DR2 |
Character(U, R) |
General character of land use surrounding the interchange, preferably based on FHWA urban area boundaries:
U = Urban
R = Rural |
Ramp type |
DR3 |
Character(ON, OFF, FWY) |
The type of ramps are distinguished by the following codes:
ON = on-ramp
OFF = off-ramp
FWY = freeway-to-freeway ramp |
Ramp configuration |
DR4 |
Character(D, PL, FFL, DIR) |
This variable defines the basic geometric design of the ramp; the abbreviated codes correspond as follows:
D = diamond ramp
PL = parclo loop ramp
FFL = free-flow loop ramp
DIR = directional ramp |
Severity level |
DR5 |
Character(TOT, FI) |
This variable identifies the crash severity level:
TOT = total crashes
FI = fatal and injury crashes |
Log intercept (a) |
DR6 |
Numeric |
Intercept of SPF. |
Coefficient of log ADT (b) |
DR7 |
Numeric |
Coefficient of log ADT (ramp) parameter. |
Coefficient of ramp length (e) |
DR8 |
Numeric |
Coefficient of ramp length parameter. |
Dispersion parameter |
DR9 |
Numeric |
Dispersion parameter associated with negative binomial regression. |
Max ADT |
DR10 |
Numeric(veh/day) |
The maximum traffic volume of the ramp for which the coefficients of the SPF apply. |
Table 19 presents the coefficients and parameters of the default SPFs for acceleration lanes provided within ISAT. Table 19 also shows the functional form of the models. Table 20 defines the coefficients and parameters of the defaults SPFs. Four SPFs for acceleration lanes are provided within ISAT and are a function of the following:
- Area type.
- Severity level.
Portions of the SPFs Accel Lanes worksheet are protected. The user may only modify the default values for the Constant (C), Log intercept (a), Coeff of ADTRamp (b), Coeff of length (c), Coeff of ADTFreeway (d), dispersion parameter, and mean length.
Table 19. SPF Coefficients and parameters for acceleration lanes.
SPF No. |
Area Type |
Severity Level |
Constant (C) |
Log Intercept (a) |
CoeffofADTRamp (b) |
Coeffof Length (c) |
CoeffofADTFreeway (d) |
Dispersion Parameter |
Mean Length |
numeric |
(R, U) |
(TOT, FI) |
numeric |
numeric |
numeric |
numeric |
numeric |
numeric |
km (mi) |
Bauer and Harwood Acceleration Lane SPF |
N = C x ea x ADTRampb x ec x ALL x ADTFreewayd |
DAL1 |
DAL2 |
DAL3 |
DAL4 |
DAL5 |
DAL6 |
DAL7 |
DAL8 |
DAL9 |
DAL10 |
1 |
R |
TOT |
0.44 |
-7.19 |
0.78 |
-2.59 |
0.13 |
0.66 |
0.16 (0.1) |
2 |
U |
TOT |
0.44 |
-6.82 |
0.78 |
-2.59 |
0.13 |
0.66 |
0.16 (0.1) |
3 |
R |
FI |
0.55 |
-10.68 |
0.91 |
-4.55 |
0.29 |
0.52 |
0.16 (0.1) |
4 |
U |
FI |
0.55 |
-10.68 |
0.91 |
-4.55 |
0.29 |
0.52 |
0.16 (0.1) |
Table 20. Definitions of coefficients and parameters of SPFs for acceleration Lanes.
Variable Name |
Variable No. |
Format |
Definition |
SPF No. |
DAL1 |
Numeric |
Each SPF in the analysis must be identified by a sequential integer, starting with 1; this variable is not used in calculations but is useful for organization. |
Area type |
DAL2 |
Character(U, R) |
General character of land use surrounding the interchange, preferably based on FHWA urban area boundaries:
U = Urban
R = Rural |
Severity level |
DAL3 |
Character(TOT, FI) |
This variable identifies the crash severity level:
TOT = total crashes
FI = fatal and injury crashes |
Constant (C) |
DAL4 |
Numeric |
This constant is provided because the SPFs were developed to predict crashes for a 3-year period. The constant scales the prediction to an annual basis. The constant also accounts for a ramp length term (i.e., the length of the ramp proper) in the original form of the SPF. |
Log intercept (a) |
DAL5 |
Numeric |
Intercept of SPF. |
Coefficient of log ADTRamp (b) |
DAL6 |
Numeric |
Coefficient of log ADT (ramp) parameter. |
Coefficient of length (c) |
DAL7 |
Numeric |
Coefficient of acceleration lane length (ALL) parameter. |
Coefficient of log ADTFreeway (d) |
DAL8 |
Numeric |
Coefficient of log ADT (freeway) parameter (this ADT is directional). |
Dispersion parameter |
DAL9 |
Numeric |
Dispersion parameter associated with negative binomial regression. |
Mean length |
DAL10 |
Numeric(mi) |
Mean length of acceleration lanes used to develop SPFs. |
Crossroad Ramp Terminal and Intersection SPFs
The default SPFs for crossroad ramp terminals and intersections provided within ISAT are located on the SPFs Ramp Terminals worksheet and are accessible only through the Go To SPFs for Ramp Terminals hyperlink on the Input-General worksheet. The default SPFs for crossroad ramp terminals and intersections were developed for use within SafetyAnalyst. Technically, these SPFs were developed using data for conventional at-grade intersections, but due to the lack of any suitable safety prediction models developed specifically for crossroad ramp terminals, these models are used to predict crashes at both crossroad ramp terminals and conventional intersections defined as part of an analysis area.
Table 21 presents the coefficients and parameters of the default SPFs for crossroad ramp terminals and intersections provided within ISAT. Table 21 also shows the functional form of the models. Table 22 defines the coefficients and parameters of the defaults SPFs. Sixteen SPFs for crossroad ramp terminals and intersections are provided within ISAT and are a function of the following:
- Area type.
- Type of traffic control.
- Number of legs.
- Severity level.
Portions of the SPFs Ramp Terminals worksheet are protected. The cells with column headings for SPF No., Area type, Type of traffic control, Number of legs, and Severity level are protected. The user may only modify the default values for the Log intercept (a), Coeff of ADTmajor rd (b), Coeff of ADToff-ramp (c), dispersion parameter, Max ADTmajor rd, and Max ADToff-ramp.
Crossroad Segment SPFs
The default SPFs for arterial crossroad segments provided within ISAT are located on the SPFs Crossroad Segments worksheet and are accessible only through the Go To SPFs for Crossroad Segments hyperlink on the Input-General worksheet. The default SPFs for arterial crossroad segments were developed for use within SafetyAnalyst. These SPFs predict the number of crashes that may occur on urban and rural arterial streets. These predictions include crashes that occur at intersections located on the crossroad roadway segments and are not related to the operation of the intersections; however the predictions do not include crashes that occur at intersections located on the crossroad roadway segments and are related to the operation of the intersections (i.e., these intersection related crashes are included in the predictions for crossroad ramp terminals and intersections).
Table 21. SPF Coefficients and parameters for crossroad ramp terminals and intersections.
SPF No. |
Area Type |
Type of Traffic Control |
Number of Legs |
Severity Level |
Log Intercept (a) |
Coeff of ADTmajor rd (b) |
Coeff ofADToff-ramp (c) |
Dispersion Parameter |
Max ADTmajor rd |
MaxADToff-ramp |
numeric |
(U, R) |
(SG, ST) |
(3, 4) |
(TOT, FI) |
numeric |
numeric |
numeric |
numeric |
(veh/day) |
(veh/day) |
SafetyAnalyst Ramp Terminal SPF |
N = e a x ADTmajor rdb x ADToff-rampc |
DRT1 |
DRT2 |
DRT3 |
DRT4 |
DRT5 |
DRT6 |
DRT7 |
DRT8 |
DRT9 |
DRT10 |
DRT11 |
1 |
R |
ST |
3 |
TOT |
-8.78 |
0.71 |
0.24 |
1.07 |
28,500 |
27,000 |
2 |
R |
SG |
3 |
TOT |
-6.57 |
0.66 |
0.20 |
0.33 |
36400 |
11500 |
3 |
R |
ST |
4 |
TOT |
-8.96 |
0.65 |
0.47 |
0.70 |
35,500 |
26,700 |
4 |
R |
SG |
4 |
TOT |
-6.57 |
0.66 |
0.20 |
0.33 |
36,400 |
11,500 |
5 |
U |
ST |
3 |
TOT |
-5.35 |
0.34 |
0.28 |
1.28 |
68,000 |
18,900 |
6 |
U |
SG |
3 |
TOT |
-9.85 |
0.97 |
0.18 |
0.23 |
50,000 |
25,807 |
7 |
U |
ST |
4 |
TOT |
-3.12 |
0.27 |
0.16 |
0.86 |
58,870 |
81,000 |
8 |
U |
SG |
4 |
TOT |
-3.47 |
0.42 |
0.14 |
0.32 |
75,000 |
81,000 |
9 |
R |
ST |
3 |
FI |
-9.35 |
0.71 |
0.21 |
1.23 |
28,500 |
27,000 |
10 |
R |
SG |
3 |
FI |
-7.83 |
0.75 |
0.14 |
0.50 |
36400 |
11500 |
11 |
R |
ST |
4 |
FI |
-9.36 |
0.66 |
0.40 |
0.00 |
35,500 |
26,700 |
12 |
R |
SG |
4 |
FI |
-7.83 |
0.75 |
0.14 |
0.50 |
36,400 |
11,500 |
13 |
U |
ST |
3 |
FI |
-8.45 |
0.49 |
0.39 |
1.23 |
68,000 |
18,900 |
14 |
U |
SG |
3 |
FI |
-10.22 |
0.91 |
0.21 |
0.27 |
50,000 |
25,807 |
15 |
U |
ST |
4 |
FI |
-4.35 |
0.29 |
0.19 |
0.99 |
58,870 |
81,000 |
16 |
U |
SG |
4 |
FI |
-5.11 |
0.49 |
0.16 |
0.30 |
75,000 |
81,000 |
Table 22. Definitions of coefficients and parameters of SPFs for crossroad ramp terminals and intersections.
Variable Name |
Variable No. |
Format |
Definition |
SPF No. |
DRT1 |
Numeric |
Each SPF included in the analysis must be identified by a sequential integer, starting with 1; this variable is not used in calculations but is useful for organization. |
Area type |
DRT2 |
Character(U, R) |
General character of land use surrounding the interchange, preferably based on FHWA urban area boundaries:
U = Urban
R = Rural |
Type of traffic control |
DRT3 |
Character(SG, ST) |
A code identifying the type of traffic control for the crossroad ramp terminal or intersection:
SG = signalized intersection
ST = STOP-control on the ramp or minor roadway; no control on the major crossroad |
Number of legs |
DRT4 |
Numeric(3,4) |
A code identifying the number of legs of the crossroad ramp terminal or intersection. In determining the number of legs, the user should consider whether it is most appropriate to treat each ramp that is served by the terminal as an individual leg.
3 = three-legs
4 = four-legs |
Severity level |
DRT5 |
Character(TOT, FI) |
A code identifying the crash severity level:
TOT = total crashes
FI = fatal and injury crashes |
Log intercept (a) |
DRT6 |
Numeric |
Intercept of SPF. |
Coefficient of log ADTmajor rd (b) |
DRT7 |
Numeric |
Coefficient of log ADT (major rd) (bi-directional) (ADTmajor rd is calculated 2 x major rd ADT [RT5]). |
Coefficient of log ADToff-ramp (c) |
DRT8 |
Numeric |
Coefficient of log ADT (off-ramp) (ADToff-ramp is minor-road or ramp ADT [RT8] when terminal type [RT11] is RT. ADToff-ramp is 2 x minor-road or ramp ADT [RT8] when terminal type [RT11] is CI.) |
Dispersion parameter |
DRT9 |
Numeric |
Dispersion parameter associated with negative binomial regression. |
Max ADTmajor rd |
DRT10 |
Numeric(veh/day) |
The maximum traffic volume on the crossroad for which the coefficients of the SPF apply. |
Max ADToff-ramp |
DRT11 |
Numeric(veh/day) |
The maximum traffic volume on the ramp for which the coefficients of the SPF apply. |
Table 23 presents the coefficients and parameters of the default SPFs for arterial crossroad segments provided within ISAT. Table 23 also shows the functional form of the models. Table 24 defines the coefficients and parameters of the defaults SPFs. Twenty SPFs for crossroad roadway segments are provided within ISAT and are a function of the following:
- Area type.
- Number of through lanes.
- Median type.
- Severity level.
Portions of the SPFs Crossroad Segments worksheet are protected. The cells with column heads for SPF No., Area type, Number of through lanes, Median, and Severity level are protected. The user may only modify the default values for the Log intercept (a), Coeff of log ADT (b), dispersion parameter, and Max ADT.
Calibration Coefficients
ISAT makes use of SPFs from previous and ongoing safety research. SPFs developed for SafetyAnalyst, and as part of research conducted by Bauer and Harwood3, are incorporated into the program. The SPFs for SafetyAnalyst are based upon data from California, Minnesota, Ohio, and Washington. Bauer and Harwood3 used data from Washington to formulate their statistical models. As such, the SPFs in ISAT are valid only for application to the States and time periods for which the models were developed. However, ISAT includes calibration coefficients that allow the SPFs developed for one particular State and one particular time period to be applied to other areas and time periods. By adjusting the calibration coefficients within ISAT based upon local crash data, the calculations within ISAT are more directly applicable to the user's own agency and more useful safety predictions are obtained, better reflecting the local safety experience.
It is recommended that one of the first steps a user should take when beginning to work with ISAT, and definitely before using ISAT to perform actual safety assessments, is modify the calibration coefficients based upon an agency's own crash data. Subsequently, these calibration coefficients should be updated on an annual basis.
Table 23. SPF coefficients and parameters for crossroad segments.
SPF No. |
Area Type |
Number of Through Lanes (Directional) |
Median |
Severity Level |
Log Intercept (a) |
Coeff of Log ADT (b) |
Dispersion Parameter |
Max ADT |
numeric |
(U, R) |
(1, 2, 3) |
(D, U) |
(TOT, FI) |
numeric |
numeric |
numeric |
(veh/day) |
SafetyAnalyst Crossroad Segment SPF |
N = ea x ADTb x SL |
DRS1 |
DRS2 |
DRS3 |
DRS4 |
DRS5 |
DRS6 |
DRS7 |
DRS8 |
DRS9 |
1 |
R |
1 |
U |
TOT |
-3.56 |
0.55 |
0.45 |
30,025 |
2 |
R |
2 |
U |
TOT |
-2.58 |
0.44 |
1.28 |
42,638 |
3 |
R |
3 |
U |
TOT |
-2.58 |
0.44 |
1.28 |
42,638 |
4 |
R |
2 |
D |
TOT |
-8.21 |
0.93 |
0.47 |
31,188 |
5 |
R |
3 |
D |
TOT |
-8.21 |
0.93 |
0.47 |
31,188 |
6 |
U |
1 |
U |
TOT |
-7.16 |
0.84 |
4.40 |
29,850 |
7 |
U |
2 |
U |
TOT |
-10.24 |
1.29 |
0.85 |
57,901 |
8 |
U |
3 |
U |
TOT |
-10.24 |
1.29 |
0.85 |
57,901 |
9 |
U |
2 |
D |
TOT |
-11.85 |
1.34 |
5.91 |
77,735 |
10 |
U |
3 |
D |
TOT |
-11.85 |
1.34 |
5.91 |
77,735 |
11 |
R |
1 |
U |
FI |
-4.89 |
0.53 |
0.45 |
30,025 |
12 |
R |
2 |
U |
FI |
-4.96 |
0.60 |
1.06 |
42,638 |
13 |
R |
3 |
U |
FI |
-4.96 |
0.60 |
1.06 |
42,638 |
14 |
R |
2 |
D |
FI |
-10.31 |
1.03 |
0.33 |
31,188 |
15 |
R |
3 |
D |
FI |
-10.31 |
1.03 |
0.33 |
31,188 |
16 |
U |
1 |
U |
FI |
-8.84 |
0.89 |
4.54 |
29,850 |
17 |
U |
2 |
U |
FI |
-12.07 |
1.39 |
0.81 |
57,901 |
18 |
U |
3 |
U |
FI |
-12.07 |
1.39 |
0.81 |
57,901 |
19 |
U |
2 |
D |
FI |
-14.87 |
1.52 |
5.81 |
77,735 |
20 |
U |
3 |
D |
FI |
-14.87 |
1.52 |
5.81 |
77,735 |
Table 24. Definitions of coefficients and parameters of SPFs for crossroad segments.
Variable Name |
Variable No. |
Format |
Definition |
SPF No. |
DRS1 |
Numeric |
Each SPF in the analysis must be identified by a sequential integer, starting with 1; this variable is not used in calculations but is useful for organization. |
Area type |
DRS2 |
Character(U, R) |
General character of land use surrounding the interchange, preferably based on FHWA urban area boundaries:
U = Urban
R = Rural |
Number of through lanes |
DRS3 |
Numeric(1, 2, 3) |
A code identifying the number of through lanes in a given direction on the crossroad segment. |
Median |
DRS4 |
Character(U, D) |
A code identifying whether a median is present on the crossroad segment:
U = the segment is undivided
D = the segment is divided |
Severity level |
DRS5 |
Character(TOT, FI) |
A code identifying the crash severity level:TOT = total crashesFI = fatal and injury crashes |
Log intercept (a) |
DRS6 |
Numeric |
Intercept of SPF. |
Coefficient of log ADT (b) |
DRS7 |
Numeric |
Coefficient of log ADT parameter (this ADT is bi-directional). |
Dispersion parameter |
DRS8 |
Numeric |
Dispersion parameter with negative binomial regression. |
Max ADT |
DRS9 |
Numeric(veh/day) |
The maximum traffic volume on the crossroad for which the coefficients of the SPF apply. |
The calibration coefficients adjust the predicted number of total and fatal and injury crashes, calculated within ISAT, to account for differences in crash patterns in different geographical areas that are not directly addressed by the SPFs. These differences may be related to differences in:
- Driver population and trip purposes.
- Climate.
- Animal populations.
- Crash reporting thresholds.
- Crash investigation practices.
The calibration coefficients are intended to account for these differences and provide crash predictions that are comparable to the estimates that a highway agency would obtain had the SPFs in ISAT been developed using their own crash records system.
Tables 25 through 28 show the specific SPFs for mainline freeway segments, ramps, crossroad ramp terminals and intersections, and crossroad roadway segments that need calibration coefficients. The nominal or default value of the calibration coefficients is shown in the tables as 1.00 for each of the SPFs. This nominal value for each SPF needs to be replaced with a calculated value appropriate for the highway agency applying the model. In general terms, calibration coefficients greater than 1.00 apply to agencies that experience more crashes than predicted by the default SPFs. Calibration coefficients less than 1.00 apply to agencies that experience fewer crashes than predicted by the default SPFs. A procedure for calculating calibration coefficients is presented below.
Calibration Procedure
This calibration procedure is applicable to each SPF (i.e., each row) in tables 25 through 28. Thus, in total this procedure should be carried out 20 times to calibrate the mainline freeway segment SPFs, 28 times to calibrate the ramp SPFs, 20 times to calibrate the crossroad ramp terminal and intersection SPFs, and 20 times to calibrate the crossroad roadway segment SPFs. The calibration procedure described below makes use of ISAT, but the procedure could be performed externally without making use of ISAT. The nomenclature used for describing the calibration procedure is defined in table 29.
Table 25. Calibration coefficients for mainline freeway segment SPFs.
CCMF1 |
CCMF2 |
CCMF3 |
CCMF4 |
CCMF5 |
CCMF6 |
SPF No. |
Area Type |
Interchange Area |
Number of Through Lanes (Directional) |
Severity Level |
Calibration Coefficients |
1 |
R |
Y |
2 |
TOT |
1.000 |
2 |
R |
Y |
3 |
TOT |
1.000 |
3 |
U |
Y |
2 |
TOT |
1.000 |
4 |
U |
Y |
3 |
TOT |
1.000 |
5 |
U |
Y |
4 |
TOT |
1.000 |
6 |
R |
Y |
2 |
FI |
1.000 |
7 |
R |
Y |
3 |
FI |
1.000 |
8 |
U |
Y |
2 |
FI |
1.000 |
9 |
U |
Y |
3 |
FI |
1.000 |
10 |
U |
Y |
4 |
FI |
1.000 |
11 |
R |
N |
2 |
TOT |
1.000 |
12 |
R |
N |
3 |
TOT |
1.000 |
13 |
U |
N |
2 |
TOT |
1.000 |
14 |
U |
N |
3 |
TOT |
1.000 |
15 |
U |
N |
4 |
TOT |
1.000 |
16
|
R
|
N
|
2
|
FI
|
1.000
|
17
|
R
|
N
|
3
|
FI
|
1.000
|
18
|
U
|
N
|
2
|
FI
|
1.000
|
19
|
U
|
N
|
3
|
FI
|
1.000
|
20
|
U
|
N
|
4
|
FI
|
1.000
|
NOTE: The default values of 1.00 for the calibration coefficients should be replaced with calculated values appropriate for specific highway agencies.
|
Table 26. Calibration coefficients for ramp SPFs.
CCR1 |
CCR2 |
CCR3 |
CCR4 |
CCR5 |
CCR6 |
SPF No. |
Area Type |
Type of Ramp |
Ramp Configuration |
Severity Level |
Calibration Coefficients |
1 |
R |
OFF |
D |
TOT |
1.000 |
2 |
R |
ON |
D |
TOT |
1.000 |
3 |
R |
OFF |
PL |
TOT |
1.000 |
4 |
R |
ON |
PL |
TOT |
1.000 |
5 |
R |
OFF |
FFL |
TOT |
1.000 |
6 |
R |
ON |
FFL |
TOT |
1.000 |
7 |
R |
FWY |
DIR |
TOT |
1.000 |
8 |
U |
OFF |
D |
TOT |
1.000 |
9 |
U |
ON |
D |
TOT |
1.000 |
10 |
U |
OFF |
PL |
TOT |
1.000 |
11 |
U |
ON |
PL |
TOT |
1.000 |
12 |
U |
OFF |
FFL |
TOT |
1.000 |
13 |
U |
ON |
FFL |
TOT |
1.000 |
14 |
U |
FWY |
DIR |
TOT |
1.000 |
15 |
R |
OFF |
D |
FI |
1.000 |
16 |
R |
ON |
D |
FI |
1.000 |
17 |
R |
OFF |
PL |
FI |
1.000 |
18 |
R |
ON |
PL |
FI |
1.000 |
19 |
R |
OFF |
FFL |
FI |
1.000 |
20 |
R |
ON |
FFL |
FI |
1.000 |
21 |
R |
FWY |
DIR |
FI |
1.000 |
22 |
U |
OFF |
D |
FI |
1.000 |
23 |
U |
ON |
D |
FI |
1.000 |
24 |
U |
OFF |
PL |
FI |
1.000 |
25 |
U |
ON |
PL |
FI |
1.000 |
26 |
U |
OFF |
FFL |
FI |
1.000 |
27 |
U |
ON |
FFL |
FI |
1.000 |
28 |
U |
FWY |
DIR |
FI |
1.000 |
NOTE: The default values of 1.00 for the calibration coefficients should be replaced with calculated values appropriate for specific highway agencies. |
Table 27. Calibration coefficients for crossroad ramp terminal and intersection SPFs.
CCRT1 |
CCRT2 |
CCRT3 |
CCRT4 |
CCRT5 |
CCRT6 |
SPF No. |
Area Type |
Type of Traffic Control |
Number of Legs |
Severity Level |
Calibration Coefficients |
1 |
R |
ST |
3 |
TOT |
1.000 |
2 |
R |
SG |
3 |
TOT |
1.000 |
3 |
R |
ST |
4 |
TOT |
1.000 |
4 |
R |
SG |
4 |
TOT |
1.000 |
5 |
U |
ST |
3 |
TOT |
1.000 |
6 |
U |
SG |
3 |
TOT |
1.000 |
7 |
U |
ST |
4 |
TOT |
1.000 |
8 |
U |
SG |
4 |
TOT |
1.000 |
9 |
R |
ST |
3 |
FI |
1.000 |
10 |
R |
SG |
3 |
FI |
1.000 |
11 |
R |
ST |
4 |
FI |
1.000 |
12 |
R |
SG |
4 |
FI |
1.000 |
13 |
U |
ST |
3 |
FI |
1.000 |
14 |
U |
SG |
3 |
FI |
1.000 |
15 |
U |
ST |
4 |
FI |
1.000 |
16 |
U |
SG |
4 |
FI |
1.000 |
Note: The default values of 1.00 for the calibration coefficients should be replaced with calculated values appropriate for specific highway agencies. |
Table 28. Calibration coefficients for crossroad segment SPFs.
CCRS1 |
CCRS2 |
CCRS3 |
CCRS4 |
CCRS5 |
CCRS6 |
SPF No. |
Area Type |
Number of Through Lanes (Directional) |
Median |
Severity Level |
Calibration Coefficients |
1 |
R |
1 |
U |
TOT |
1.000 |
2 |
R |
2 |
U |
TOT |
1.000 |
3 |
R |
3 |
U |
TOT |
1.000 |
4 |
R |
2 |
D |
TOT |
1.000 |
5 |
R |
3 |
D |
TOT |
1.000 |
6 |
U |
1 |
U |
TOT |
1.000 |
7 |
U |
2 |
U |
TOT |
1.000 |
8 |
U |
3 |
U |
TOT |
1.000 |
9 |
U |
2 |
D |
TOT |
1.000 |
10 |
U |
3 |
D |
TOT |
1.000 |
11 |
R |
1 |
U |
FI |
1.000 |
12 |
R |
2 |
U |
FI |
1.000 |
13 |
R |
3 |
U |
FI |
1.000 |
14 |
R |
2 |
D |
FI |
1.000 |
15 |
R |
3 |
D |
FI |
1.000 |
16 |
U |
1 |
U |
FI |
1.000 |
17 |
U |
2 |
U |
FI |
1.000 |
18 |
U |
3 |
U |
FI |
1.000 |
19 |
U |
2 |
D |
FI |
1.000 |
20 |
U |
3 |
D |
FI |
1.000 |
NOTE: The default values of 1.00 for the calibration coefficients should be replaced with calculated values appropriate for specific highway agencies. |
Table 29. Nomenclature for calibration procedures.
Term |
Explanation |
i |
Subscript to represent site i |
y |
Subscript to represent the year y
The first year of the analysis period is considered Year 1 (i.e., y=1)
The last year of the analysis period is considered Year Y (i.e., y=Y) |
TOT |
Subscript to denote total crashes |
FI |
Subscript to denote fatal and injury crashes |
Oiy |
Observed number of crashes at site i during year y |
O |
Total number of observed crashes for the entire analysis period across all individual sites of the interchange element of interest |
Niy |
Predicted number of crashes at a site i during year y |
NA |
Total number of predicted crashes for the entire analysis period across all individual sites of the interchange element of interest |
C |
Calibration coefficient |
Step 1. Select sites for use in applying the calibration procedures.
Select up to a maximum of 50 sites of the same type for use in applying the calibration procedures. There is no specific number of sites that must be selected in this step, in part because the number of sites that should be included is dependent upon the next step (i.e., select analysis years). As a general rule of thumb, at least 20 sites are desirable, and selecting more sites should increase the accuracy of the predictions output by ISAT.
Step 2. Select the analysis years for applying the calibration procedures.
Two approaches could be considered when performing this step. One approach would be to limit the analysis period to the most recent year for which a full calendar year of crash data are available. If this approach is taken, the calibration coefficients and calculations will specifically reflect the crash trends of the selected calendar year.
The second approach is to select multiple years for the analysis period. It is recommended that up to the 10 most recent years for which crash data are available for the selected sites be included in the analysis. If this approach is taken, the calibration coefficients and calculations will not specifically reflect crash trends for the most recent year of crash data, but rather reflect the average crash trend over the collective analysis years. This second approach (i.e., multiple years) is preferred over calibrating the SPFs based upon the most recent year crash data.
A couple of issues should be considered collectively when selecting sites and analysis years on which to base the calibration coefficients:
- The sites should not have undergone reconstruction at any point during this time period.
- The analysis period has to be same for all sites.
Step 3. Predict the total number of crashes across the selected sites and analysis years.
On the Input-General worksheet, the user will input data for variables G1 through G4. The beginning and ending year of analysis coincide with the analysis years as selected in Step 2. For variable G5 (i.e., crash data available), a value of "N" should be provided for each primary element (i.e., mainline freeway segments, ramps, crossroad ramp terminals and intersections, and crossroad segments).
On the respective input worksheet either for mainline freeway segments, ramps, crossroad ramp terminals and intersections, or crossroad segments, the user should input data for the sites selected in Step 1.
As part of Step 3, the calibration coefficient for the respective site subtype should be modified to the default value of 1.00.
Having entered all of the input data, the user can click the Perform Calculations button. The table for number of predicted crashes by interchange element type will show the number of predicted crashes for the respective interchange element type for both total and fatal and injury crash levels. Because data were input for only one subtype of the given interchange element, the predicted values are applicable to the given subtype, and these values are truly predicted values because no crash data were considered in the analysis. This calculation is represented as:
(Eq. 1)
Step 4. Determine the total number of observed crashes across the selected sites and analysis years.
This step is performed externally to ISAT and is represented as:
(Eq. 2)
When determining the number of observed crashes across selected sites, the same rules for attributing crashes to particular interchange element types should be followed as explained in the General Interchange Inputs section of this manual. The rules are summarized here as follows:
- Crashes that occur along or within mainline freeway segments, deceleration lanes, and acceleration lanes should be attributed to mainline freeway segments.
- All crashes that occur within 76.2 m (250 ft) of a crossroad ramp terminal or intersection and are classified as intersection-related should be attributed to crossroad ramp terminals and intersections. All crashes that occur within 76.2 m (250 ft) of a crossroad ramp terminal or intersection but are not classified as intersection-related should be attributed either to ramps or crossroad roadway segments.
- All crashes that occur along the ramp proper portion of an interchange ramp should be attributed to ramps. For crashes that occur on the ramp proper and are within 76.2 m (250 ft) of the crossroad ramp terminal, if the crash is related to the operation of the ramp terminal (i.e., intersection-related), then the crash should be attributed to the crossroad ramp terminal, but if the crash is not related to the operation of the ramp terminal, then the crash should be attributed to the ramp.
- Crashes that occur along or within arterial crossroad roadway segments should be attributed as such, except those crashes that occur within 76.2 m (250 ft) of a ramp terminal or intersection and are intersection-related, in which case the crashes should be attributed to crossroad ramp terminals and intersections.
Step 5. Compute the calibration coefficient such that:
(Eq. 3)
This is the value of the calibration coefficient that would be entered into the calibration table for the respective SPF.
Calibration Example
This example illustrates the general calibration procedure for calculating the calibration coefficient for the mainline freeway segment SPF No. 1. Table 15 shows that SPF No. 1 for mainline freeway segments is used to predict crash frequencies for the following situation:
Interchange element:
|
Mainline freeway segment
|
Area type:
|
Rural
|
Interchange area:
|
Yes
|
Number of through lanes (directional):
|
2
|
Severity level:
|
TOT
|
Step 1. Select sites for use in applying the calibration procedures.
In this hypothetical example, 20 sites were selected (i.e., 10 sites on I-80 and 10 sites on
I-79).
Step 2. Select the analysis years for applying the calibration procedures.
Five years of crash data were used, from 2001 through 2005.
Step 3. Predict the total number of crashes across the selected sites and analysis years.
Table 30 shows the type of data input on the Input-General worksheet. In this case, descriptive data are input for variables G1 through G4, and when asked about the availability of crash data, the input is "N" even though crash data are actually available. If crash data were entered on the Input-General worksheet, then the output would be an EB-adjusted expected value representing a weighted average of the predicted values from the SPF and the observed crash totals. By not including the crash data in these calculations, the output results represent predicted values using mainline freeway segment SPF No. 1.
Table 31 shows the type of data to be entered on the Input Mainline Freeways worksheet. Data for 20 sites are entered into this worksheet. To reiterate, these sites should all be of the same type. In this example, these sites are rural mainline freeway segments within interchange area that have 2 through lanes in the designated direction.
Having entered all of the input data on the Input-General and Input Mainline Freeways worksheets, the calculations are performed by clicking the Perform Calculations button on the Input-General worksheet. Table 32 shows a portion of the output report generated for this example. In this case the table of interest is the number of predicted crashes by interchange element type which shows the number of predicted crashes for the four interchange element types. In this example data were input only for mainline freeway segments. Table 32 indicates that 55.1 total crashes were predicted to occur on these 20 sites during the 5 year period from 2001 to 2005.
(Eq. 4)
Step 4. Determine the total number of observed crashes across the selected sites and analysis years.
Assume that 60 total crashes were observed at these 20 sites during the period from 2001 through 2005.
(Eq. 5)
Step 5. Compute the calibration coefficient.
Plugging the predicted and observed values into equation 3 yields:
(Eq. 6)
This value would then be entered into the calibration table for mainline freeway segments SPF No. 1 as illustrated in table 33. This procedure would be repeated for each SPF for mainline freeway segments, and the resulting calculated value would be entered as appropriate into the calibration table. Similarly, this procedure would be repeated for each SPF for ramps, crossroad ramp terminals and intersections, and crossroad roadway segments.
Table 30. Example data for calibration procedure (Input-General).
GI1 |
Project description |
character |
MF (calibration) |
|
|
|
GI2 |
Analyst |
|
MRI |
|
|
|
GI3 |
Date |
|
2/20/2007 |
|
|
|
GI4 |
Area type |
(U,R) |
R |
|
|
|
GI5 |
Beginning year of analysis |
numeric |
2001 |
|
|
|
GI6 |
Ending year of analysis |
numeric |
2005 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Mainline Freeways |
Ramps |
Crossroad Ramp Terminals and Intersections |
Crossroad Segments |
GI7 |
Crash data available? |
(Y, N) |
N |
N |
N |
N |
GI8 |
Beginning year of crash data |
numeric |
|
|
|
|
GI9 |
Ending year of crash data |
numeric |
|
|
|
|
GI10 |
Observed number of crashes |
numeric |
|
|
|
|
Table 31. Example data for calibration procedure (Input Mainline Freeways).
MF1 |
MF2 |
MF3 |
MF4 |
MF5 |
MF6 |
MF7 |
MF8 |
MF9 |
MF10 |
MF11 |
Segment Number |
Segment Description |
Direction of Travel |
Beginning MP |
Ending MP |
LengthofSegment |
Number of Through Lanes (Directional) |
Mainline ADT (Directional) |
Mainline ADT Year |
ADT Growth Rate |
Within Interchange Area? |
numeric |
character |
(NB, SB,EB, WB) |
numeric |
numeric |
km (mi) |
(2, 3, 4) |
(veh/day) |
numeric |
(percent/ year) |
(Y, N) |
1 |
EB I-80 |
EB |
1.000 |
1.350 |
0.350 |
2 |
4,000 |
2004 |
2.0 |
Y |
2 |
EB I-80 |
EB |
10.000 |
10.400 |
0.400 |
2 |
4,000 |
2004 |
2.0 |
Y |
3 |
EB I-80 |
EB |
50.000 |
50.500 |
0.500 |
2 |
3,500 |
2004 |
2.0 |
Y |
4 |
EB I-80 |
EB |
100.000 |
100.400 |
0.400 |
2 |
4,500 |
2004 |
2.0 |
Y |
5 |
EB I-80 |
EB |
150.000 |
150.500 |
0.500 |
2 |
4,500 |
2004 |
2.0 |
Y |
6 |
WB I-80 |
WB |
25.000 |
25.500 |
0.500 |
2 |
4,500 |
2004 |
2.0 |
Y |
7 |
WB I-80 |
WB |
75.000 |
75.400 |
0.400 |
2 |
4,500 |
2004 |
2.0 |
Y |
8 |
WB I-80 |
WB |
125.000 |
125.400 |
0.400 |
2 |
3,500 |
2004 |
2.0 |
Y |
9 |
WB I-80 |
WB |
175.000 |
175.500 |
0.500 |
2 |
4,000 |
2004 |
2.0 |
Y |
10 |
WB I-80 |
WB |
225.000 |
225.350 |
0.350 |
2 |
4,000 |
2004 |
2.0 |
Y |
11 |
NB I-79 |
NB |
5.000 |
5.500 |
0.500 |
2 |
3,000 |
2005 |
2.0 |
Y |
12 |
NB I-79 |
NB |
40.000 |
40.400 |
0.400 |
2 |
3,250 |
2005 |
2.0 |
Y |
13 |
NB I-79 |
NB |
80.000 |
80.500 |
0.500 |
2 |
4,000 |
2005 |
2.0 |
Y |
14 |
NB I-79 |
NB |
120.000 |
120.500 |
0.500 |
2 |
3,700 |
2005 |
2.0 |
Y |
15 |
NB I-79 |
NB |
170.000 |
170.400 |
0.400 |
2 |
3,000 |
2005 |
2.0 |
Y |
16 |
SB I-79 |
SB |
10.000 |
10.400 |
0.400 |
2 |
4,000 |
2005 |
2.0 |
Y |
17 |
SB I-79 |
SB |
60.000 |
60.350 |
0.350 |
2 |
4,250 |
2005 |
2.0 |
Y |
18 |
SB I-79 |
SB |
130.000 |
130.350 |
0.350 |
2 |
4,500 |
2005 |
2.0 |
Y |
19 |
SB I-79 |
SB |
180.000 |
180.400 |
0.400 |
2 |
4,000 |
2005 |
2.0 |
Y |
20 |
SB I-79 |
SB |
200.000 |
200.500 |
0.500 |
2 |
3,800 |
2005 |
2.0 |
Y |
Table 32. Example output report for calibration procedure.
General Interchange Information |
|
|
|
Project description: |
MF (calibration) |
|
|
|
Analyst: |
MRI |
|
|
|
Date: |
2/20/2007 |
|
|
|
Area type: |
Rural |
|
|
|
Beginning year of analysis period: |
2001 |
|
|
|
Ending year of analysis period: |
2005 |
|
|
|
|
|
Mainline Freeway Segments |
Ramps |
Crossroad Ramp Terminals and Intersections |
Crossroad Segments |
|
|
|
Crash data available: |
N |
N |
N |
N |
|
|
|
Beginning year of crash data: |
|
|
|
|
|
|
|
Ending year of crash data: |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Number of Predicted Crashes for Entire Interchange Area |
|
|
|
Number of Predicted Crashes During Analysis Period |
Average Number of Predicted Crashes per Year During Analysis Period |
|
|
|
|
|
|
Total |
FI |
PDO |
Total |
FI |
PDO |
|
|
|
55.1 |
16.3 |
38.9 |
11.0 |
3.3 |
7.8 |
|
|
|
|
|
|
|
|
|
|
|
|
Number of Predicted Crashes by Interchange Element Type |
|
Interchange Element Type |
Number of Sites |
Number of Predicted Crashes During Analysis Period |
MVMT |
MEV |
Crash Rate |
Total |
FI |
PDO |
(per MVMT or MEV) |
Mainline freeway segments |
20 |
55.1 |
16.3 |
38.9 |
59.691 |
|
0.923 |
Ramps |
0 |
|
|
|
|
|
|
Crossroad ramp terminals & ints |
0 |
|
|
|
|
|
|
Crossroad segments |
0 |
|
|
|
|
|
|
Total |
20 |
55.1 |
16.3 |
38.9 |
59.691 |
|
0.923 |
Table 33. Example calibration procedure (Input Calibration Worksheet).
CCMF1 |
CCMF2 |
CCMF3 |
CCMF4 |
CCMF5 |
CCMF6 |
SPF No. |
Area Type |
Interchange Area |
Number of Through Lanes (Directional) |
Severity Level |
Calibration Coefficients |
1 |
R |
Y |
2 |
TOT |
1.089 |
2 |
R |
Y |
3 |
TOT |
|
3 |
U |
Y |
2 |
TOT |
|
4 |
U |
Y |
3 |
TOT |
|
5 |
U |
Y |
4 |
TOT |
|
6 |
R |
Y |
2 |
FI |
|
7 |
R |
Y |
3 |
FI |
|
8 |
U |
Y |
2 |
FI |
|
9 |
U |
Y |
3 |
FI |
|
10 |
U |
Y |
4 |
FI |
|
11 |
R |
N |
2 |
TOT |
|
12 |
R |
N |
3 |
TOT |
|
13 |
U |
N |
2 |
TOT |
|
14 |
U |
N |
3 |
TOT |
|
15 |
U |
N |
4 |
TOT |
|
16 |
R |
N |
2 |
FI |
|
17 |
R |
N |
3 |
FI |
|
18 |
U |
N |
2 |
FI |
|
19 |
U |
N |
3 |
FI |
|
20 |
U |
N |
4 |
FI |
|
Three comments concerning this procedure are worth noting:
- SPFs for a particular site subtype are provided for two severity levels, total and fatal and injuries. The output from Step 3 provides predicted values for both severity levels. In the example above, the predicted number of fatal and injury crashes for the 20 sites is 16.3 crashes for the 5 year period (Table 32). Thus, the output for a particular subtype of interchange element could be used in calculating the calibration coefficients for both SPFs (i.e., TOT and FI).
- In terms of efficiency, it is possible that when using ISAT to calculate the predicted values for calibration purposes, data could be entered for 4 subtypes of interchange elements as long as they were for different types of elements (i.e., mainline freeway segments, ramps, crossroad ramp terminals and intersections, and crossroad roadway segments). In this case, the output report could be used for calculating the calibration coefficients for up to 8 SPFs (i.e., 2 for each interchange element) in one iteration.
- In Step 3 it is noted that when performing the calibration procedures, the calibration coefficient for the respective site subtype should be the default value of 1.00. This is important so that the predicted values from the SPFs (i.e., NA) are not scaled/adjusted prior to calculating the calibration coefficient.
Crash Distributions by Severity and Type
This section provides the default distributions of crash severity and crash type for specific types of interchange elements. The default distributions are based upon crash data from Washington State. The crash distributions are used to predict crash frequencies for 3 severity levels (i.e., TOT, FI, and PDO) and 13 crash types. The crash types are classified as either single-vehicle crashes (SV) or multiple-vehicle crashes (MV). In general terms these crash proportions are multiplied by the predicted or estimated values for total and fatal and injury crashes to obtain the number of crashes of a specified collision type and severity. The number of PDO crashes is estimated by subtracting fatal and injury crashes from total crashes.
It is recommended that one of the first steps a user should do when beginning to work with ISAT, and definitely before using ISAT to perform actual safety assessments, is modify the crash distributions based upon an agency's own crash data. Subsequently, these crash distributions should be updated on an annual basis.
The crash proportions are calculated as follows. The calculations apply to a given type of site and severity level. The classifications of subtypes for which crash distributions are necessary are somewhat different than the subtypes of SPFs for the respective interchange elements. In some cases, multiple subtypes are aggregated together for calculating crash proportions so the crash proportions are applied to multiple subtypes of interchange elements. In calculating the crash type proportions, it is assumed that crash data are available for Y years at I sites of a given subtype in a given jurisdiction (e.g., State). The terms in the calculations are defined in table 34. The proportions, P(CT/TOT) and P(CT/FI), are computed across all sites I of a given subtype as follows:
Table 34. Nomenclature for crash distribution calculations.
Term |
Explanation |
i |
Subscript to represent site i |
y |
Subscript to represent the year y
The first year of the analysis period is considered Year 1 (i.e., y=1)
The last year of the analysis period is considered Year Y (i.e., y=Y) |
TOT |
Subscript to denote total crashes |
FI |
Subscript to denote fatal and injury crashes |
CT |
Subscript to denote crash type |
Oiy |
Observed number of crashes at site i during year y |
P(CT/TOT) |
Calculated proportion of crashes of the crash type of interest for all TOT crashes for all years of available data |
P(CT/FI) |
Calculated proportion of crashes of the crash type of interest for all FI crashes for all years of available data |
(Eq 7)
(Eq. 8)
The default distributions provided within ISAT for total and fatal and injury crashes are the same. This is not a requirement of the program. It is up to the user to determine whether or not providing the actual proportions for the two severity levels is desirable.
Crash Distributions for Mainline Freeways
The default crash distributions for mainline freeway segments provided within ISAT are located on the Distributions Mainline Freeways worksheet and are accessible only through the Go To Distributions for Mainline Freeways hyperlink on the Input-General worksheet. Table 35 illustrates the crash distributions for mainline freeways. For a given group of crash types (i.e., 13 crash types) and subtype, the sum of the proportions should equal 1.00.
Portions of the Distributions Mainline Freeways worksheet are protected. The user may only modify the default values for the mainline freeway segment subtypes (i.e., mainline freeway outside interchange area and mainline freeway within interchange area).
Crash Distributions for Ramps
The default crash distributions for ramps provided within ISAT are located on the Distributions Ramps worksheet and are accessible only through the Go To Distributions for Ramps hyperlink on the Input-General worksheet. Table 36 illustrates the crash distributions for ramps. For a given group of crash types (i.e., 13 crash types) and subtype, the sum of the proportions should equal 1.00.
Portions of the Distributions Ramps worksheet are protected. The user may only modify the default values for the ramp subtypes (i.e., diamond off-ramp, diamond on-ramp, parclo off-ramp, parlco on-ramp, free-flow off-ramp, free-flow on-ramp, and directional ramp).
Table 35. Proportions of crashes by severity and type for mainline freeways.
Crash No. |
Crash Type |
Number of Vehicles Involved |
Area Type |
Severity Level |
Mainline Freeway Outside Interchange Area |
Mainline Freeway Within Interchange Area |
|
|
(SV, MV) |
(R, U) |
(TOT, FI) |
numeric |
numeric |
CPF1 |
CPF2 |
CPF3 |
CPF4 |
CPF5 |
CPF6 |
CPF7 |
1 |
Fixed object |
SV |
R |
TOT |
0.176 |
0.266 |
2 |
Animal |
SV |
R |
TOT |
0.206 |
0.018 |
3 |
Pedestrian |
SV |
R |
TOT |
0.001 |
0.000 |
4 |
Bicyclist |
SV |
R |
TOT |
0.000 |
0.000 |
5 |
Parked car |
SV |
R |
TOT |
0.013 |
0.000 |
6 |
Overturn |
SV |
R |
TOT |
0.295 |
0.108 |
7 |
Other single-vehicle |
SV |
R |
TOT |
0.086 |
0.149 |
8 |
Rear-End |
MV |
R |
TOT |
0.115 |
0.225 |
9 |
Head-On |
MV |
R |
TOT |
0.006 |
0.006 |
10 |
Angle |
MV |
R |
TOT |
0.012 |
0.020 |
11 |
Sideswipe, same direction |
MV |
R |
TOT |
0.052 |
0.113 |
12 |
Sideswipe, opposite direction |
MV |
R |
TOT |
0.004 |
0.004 |
13 |
Other multiple-vehicle |
MV |
R |
TOT |
0.034 |
0.091 |
1 |
Fixed object |
SV |
U |
TOT |
0.223 |
0.237 |
2 |
Animal |
SV |
U |
TOT |
0.013 |
0.009 |
3 |
Pedestrian |
SV |
U |
TOT |
0.009 |
0.003 |
4 |
Bicyclist |
SV |
U |
TOT |
0.003 |
0.001 |
5 |
Parked car |
SV |
U |
TOT |
0.018 |
0.012 |
6 |
Overturn |
SV |
U |
TOT |
0.052 |
0.047 |
7 |
Other single-vehicle |
SV |
U |
TOT |
0.016 |
0.021 |
8 |
Rear-End |
MV |
U |
TOT |
0.410 |
0.462 |
9 |
Head-On |
MV |
U |
TOT |
0.003 |
0.002 |
10 |
Angle |
MV |
U |
TOT |
0.083 |
0.017 |
11 |
Sideswipe, same direction |
MV |
U |
TOT |
0.113 |
0.138 |
12 |
Sideswipe, opposite direction |
MV |
U |
TOT |
0.007 |
0.003 |
13 |
Other multiple-vehicle |
MV |
U |
TOT |
0.050 |
0.048 |
1 |
Fixed object |
SV |
R |
FI |
0.176 |
0.266 |
2 |
Animal |
SV |
R |
FI |
0.206 |
0.018 |
3 |
Pedestrian |
SV |
R |
FI |
0.001 |
0.000 |
4 |
Bicyclist |
SV |
R |
FI |
0.000 |
0.000 |
5 |
Parked car |
SV |
R |
FI |
0.013 |
0.000 |
6 |
Overturn |
SV |
R |
FI |
0.295 |
0.108 |
7 |
Other single-vehicle |
SV |
R |
FI |
0.086 |
0.149 |
8 |
Rear-End |
MV |
R |
FI |
0.115 |
0.225 |
9 |
Head-On |
MV |
R |
FI |
0.006 |
0.006 |
10 |
Angle |
MV |
R |
FI |
0.012 |
0.020 |
11 |
Sideswipe, same direction |
MV |
R |
FI |
0.052 |
0.113 |
12 |
Sideswipe, opposite direction |
MV |
R |
FI |
0.004 |
0.004 |
13 |
Other multiple-vehicle |
MV |
R |
FI |
0.034 |
0.091 |
Table 35. Proportions of crashes by severity and type for mainline freeways. (Continued)
Crash No. |
Crash Type |
Number of Vehicles Involved |
Area Type |
Severity Level |
Mainline Freeway Outside Interchange Area |
Mainline Freeway Within Interchange Area |
|
|
(SV, MV)
|
(R, U)
|
(TOT, FI)
|
numeric
|
numeric
|
CPF1 |
CPF2 |
CPF3 |
CPF4 |
CPF5 |
CPF6 |
CPF7 |
1
|
Fixed object
|
SV
|
U
|
FI
|
0.223
|
0.237
|
2
|
Animal
|
SV
|
U
|
FI
|
0.013
|
0.009
|
3
|
Pedestrian
|
SV
|
U
|
FI
|
0.009
|
0.003
|
4
|
Bicyclist
|
SV
|
U
|
FI
|
0.003
|
0.001
|
5
|
Parked car
|
SV
|
U
|
FI
|
0.018
|
0.012
|
6
|
Overturn
|
SV
|
U
|
FI
|
0.052
|
0.047
|
7
|
Other single-vehicle
|
SV
|
U
|
FI
|
0.016
|
0.021
|
8 |
Rear-End |
MV |
U |
FI |
0.410 |
0.462 |
9 |
Head-On |
MV |
U |
FI |
0.003 |
0.002 |
10 |
Angle |
MV |
U |
FI |
0.083 |
0.017 |
11 |
Sideswipe, same direction |
MV |
U |
FI |
0.113 |
0.138 |
12 |
Sideswipe, opposite direction |
MV |
U |
FI |
0.007 |
0.003 |
13 |
Other multiple-vehicle |
MV |
U |
FI |
0.050 |
0.048 |
Crash Distributions for Crossroad Ramp Terminals and Intersections
The default crash distributions for crossroad ramp terminals and intersections provided within ISAT are located on the Distributions Ramp Terminals worksheet and are accessible only through the Go To Distributions for Ramp Terminals hyperlink on the Input-General worksheet. Table 37 illustrates the crash distributions for crossroad ramp terminals and intersections. For a given group of crash types (i.e., 13 crash types) and subtype, the sum of the proportions should equal 1.00.
Portions of the Distributions Ramp Terminals worksheet are protected. The user may only modify the default values for the crossroad ramp terminal and intersection subtypes (i.e., 3-leg STOP-controlled [3ST], 4-leg STOP-controlled [4ST], 3-leg signalized [3SG], and 4-leg signalized [4SG]).
Crash Distributions for Crossroad Segments
The default crash distributions for crossroad segments provided within ISAT are located on the Distributions Crossroad Segment worksheet and are accessible only through the Go To Distributions for Crossroad Segments hyperlink on the Input-General worksheet. Table 38 illustrates the crash distributions for crossroad segments. For a given group of crash types (i.e., 13 crash types) and subtype, the sum of the proportions should equal 1.00.
Portions of the Distributions Crossroad Segment worksheet are protected. The user may only modify the default values for the arterial crossroad roadway segment subtypes (i.e., 1-lane undivided, 2-lane undivided, 3-lane undivided, 2-lane divided, and 3-lane divided).
Table 36. Proportions of crashes by severity and type for ramps.
Crash No. |
Crash Type |
Number of Vehicles Involved |
Area Type |
Severity Level |
Diamond Off-Ramp |
Diamond On-Ramp |
Parclo Off-Ramp |
Parclo On-Ramp |
Free-Flow Off-Ramp |
Free-Flow On-Ramp |
Directional Ramp |
|
|
(SV, MV)
|
(R, U)
|
(TOT, FI)
|
numeric
|
numeric
|
numeric
|
numeric
|
numeric
|
numeric
|
numeric
|
CPR1
|
CPR2
|
CPR3
|
CPR4
|
CPR5
|
CPR6
|
CPR7
|
CPR8
|
CPR9
|
CPR10
|
CPR11
|
CPR12
|
1 |
Fixed object |
SV |
R |
TOT |
0.215 |
0.383 |
0.292 |
0.250 |
0.286 |
0.355 |
0.397 |
2 |
Animal |
SV |
R |
TOT |
0.005 |
0.005 |
0.000 |
0.000 |
0.000 |
0.000 |
0.005 |
3 |
Pedestrian |
SV |
R |
TOT |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
4 |
Bicyclist |
SV |
R |
TOT |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
5 |
Parked car |
SV |
R |
TOT |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
6 |
Overturn |
SV |
R |
TOT |
0.055 |
0.137 |
0.092 |
0.295 |
0.357 |
0.370 |
0.107 |
7 |
Other single-vehicle |
SV |
R |
TOT |
0.103 |
0.178 |
0.092 |
0.136 |
0.143 |
0.171 |
0.184 |
8 |
Rear-End |
MV |
R |
TOT |
0.476 |
0.150 |
0.432 |
0.091 |
0.143 |
0.026 |
0.179 |
9 |
Head-On |
MV |
R |
TOT |
0.002 |
0.000 |
0.000 |
0.023 |
0.000 |
0.000 |
0.010 |
10 |
Angle |
MV |
R |
TOT |
0.017 |
0.016 |
0.031 |
0.045 |
0.071 |
0.013 |
0.013 |
11 |
Sideswipe, same direction |
MV |
R |
TOT |
0.074 |
0.068 |
0.015 |
0.023 |
0.000 |
0.039 |
0.054 |
12 |
Sideswipe, opposite direction |
MV |
R |
TOT |
0.006 |
0.000 |
0.000 |
0.023 |
0.000 |
0.026 |
0.005 |
13 |
Other multiple-vehicle |
MV |
R |
TOT |
0.047 |
0.063 |
0.046 |
0.114 |
0.000 |
0.000 |
0.046 |
1 |
Fixed object |
SV |
U |
TOT |
0.150 |
0.206 |
0.280 |
0.225 |
0.250 |
0.292 |
0.294 |
2 |
Animal |
SV |
U |
TOT |
0.001 |
0.000 |
0.001 |
0.000 |
0.000 |
0.000 |
0.000 |
3 |
Pedestrian |
SV |
U |
TOT |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
4 |
Bicyclist |
SV |
U |
TOT |
0.000 |
0.000 |
0.000 |
0.001 |
0.000 |
0.000 |
0.000 |
5 |
Parked car |
SV |
U |
TOT |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
6 |
Overturn |
SV |
U |
TOT |
0.016 |
0.036 |
0.063 |
0.057 |
0.040 |
0.051 |
0.033 |
7 |
Other single-vehicle |
SV |
U |
TOT |
0.056 |
0.077 |
0.097 |
0.080 |
0.082 |
0.100 |
0.108 |
8 |
Rear-End |
MV |
U |
TOT |
0.614 |
0.421 |
0.366 |
0.347 |
0.518 |
0.381 |
0.362 |
9 |
Head-On |
MV |
U |
TOT |
0.002 |
0.003 |
0.016 |
0.007 |
0.004 |
0.003 |
0.002 |
10 |
Angle |
MV |
U |
TOT |
0.009 |
0.020 |
0.018 |
0.039 |
0.010 |
0.015 |
0.014 |
11 |
Sideswipe, same direction |
MV |
U |
TOT |
0.084 |
0.176 |
0.086 |
0.193 |
0.061 |
0.109 |
0.128 |
12 |
Sideswipe, opposite direction |
MV |
U |
TOT |
0.003 |
0.011 |
0.012 |
0.008 |
0.003 |
0.005 |
0.004 |
13 |
Other multiple-vehicle |
MV |
U |
TOT |
0.065 |
0.050 |
0.061 |
0.043 |
0.032 |
0.044 |
0.055 |
Table 36. Proportions of crashes by severity and type for ramps. (Continued)
Crash No. |
Crash Type |
Number of Vehicles Involved |
Area Type |
Severity Level |
Diamond Off-Ramp |
Diamond On-Ramp |
Parclo Off-Ramp |
Parclo On-Ramp |
Free-Flow Off-Ramp |
Free-Flow On-Ramp |
Directional Ramp |
|
|
(SV, MV)
|
(R, U)
|
(TOT, FI)
|
numeric
|
numeric
|
numeric
|
numeric
|
numeric
|
numeric
|
numeric
|
CPR1 |
CPR2 |
CPR3 |
CPR4 |
CPR5 |
CPR6 |
CPR7 |
CPR8 |
CPR9 |
CPR10 |
CPR11 |
CPR12 |
1 |
Fixed object |
SV |
R |
FI |
0.215 |
0.383 |
0.292 |
0.250 |
0.286 |
0.355 |
0.397 |
2 |
Animal |
SV |
R |
FI |
0.005 |
0.005 |
0.000 |
0.000 |
0.000 |
0.000 |
0.005 |
3 |
Pedestrian |
SV |
R |
FI |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
4 |
Bicyclist |
SV |
R |
FI |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
5 |
Parked car |
SV |
R |
FI |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
6 |
Overturn |
SV |
R |
FI |
0.055 |
0.137 |
0.092 |
0.295 |
0.357 |
0.370 |
0.107 |
7 |
Other single-vehicle |
SV |
R |
FI |
0.103 |
0.178 |
0.092 |
0.136 |
0.143 |
0.171 |
0.184 |
8 |
Rear-End |
MV |
R |
FI |
0.476 |
0.150 |
0.432 |
0.091 |
0.143 |
0.026 |
0.179 |
9 |
Head-On |
MV |
R |
FI |
0.002 |
0.000 |
0.000 |
0.023 |
0.000 |
0.000 |
0.010 |
10 |
Angle |
MV |
R |
FI |
0.017 |
0.016 |
0.031 |
0.045 |
0.071 |
0.013 |
0.013 |
11 |
Sideswipe, same direction |
MV |
R |
FI |
0.074 |
0.068 |
0.015 |
0.023 |
0.000 |
0.039 |
0.054 |
12 |
Sideswipe, opposite direction |
MV |
R |
FI |
0.006 |
0.000 |
0.000 |
0.023 |
0.000 |
0.026 |
0.005 |
13 |
Other multiple-vehicle |
MV |
R |
FI |
0.047 |
0.063 |
0.046 |
0.114 |
0.000 |
0.000 |
0.046 |
1 |
Fixed object |
SV |
U |
FI |
0.150 |
0.206 |
0.280 |
0.225 |
0.250 |
0.292 |
0.294 |
2 |
Animal |
SV |
U |
FI |
0.001 |
0.000 |
0.001 |
0.000 |
0.000 |
0.000 |
0.000 |
3 |
Pedestrian |
SV |
U |
FI |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
4 |
Bicyclist |
SV |
U |
FI |
0.000 |
0.000 |
0.000 |
0.001 |
0.000 |
0.000 |
0.000 |
5 |
Parked car |
SV |
U |
FI |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
6 |
Overturn |
SV |
U |
FI |
0.016 |
0.036 |
0.063 |
0.057 |
0.040 |
0.051 |
0.033 |
7 |
Other single-vehicle |
SV |
U |
FI |
0.056 |
0.077 |
0.097 |
0.080 |
0.082 |
0.100 |
0.108 |
8 |
Rear-End |
MV |
U |
FI |
0.614 |
0.421 |
0.366 |
0.347 |
0.518 |
0.381 |
0.362 |
9 |
Head-On |
MV |
U |
FI |
0.002 |
0.003 |
0.016 |
0.007 |
0.004 |
0.003 |
0.002 |
10 |
Angle |
MV |
U |
FI |
0.009 |
0.020 |
0.018 |
0.039 |
0.010 |
0.015 |
0.014 |
11 |
Sideswipe, same direction |
MV |
U |
FI |
0.084 |
0.176 |
0.086 |
0.193 |
0.061 |
0.109 |
0.128 |
12 |
Sideswipe, opposite direction |
MV |
U |
FI |
0.003 |
0.011 |
0.012 |
0.008 |
0.003 |
0.005 |
0.004 |
13 |
Other multiple-vehicle |
MV |
U |
FI |
0.065 |
0.050 |
0.061 |
0.043 |
0.032 |
0.044 |
0.055 |
Table 37. Proportions of crashes by severity and type for crossroad ramp terminals and intersections.
Crash No. |
Crash Type |
Number of Vehicles Involved |
Area Type |
Severity Level |
3ST |
4ST |
3SG |
4SG |
|
|
(SV, MV)
|
(R, U)
|
(TOT, FI)
|
numeric
|
numeric
|
numeric
|
numeric
|
CPT1 |
CPT2 |
CPT3 |
CPT4 |
CPT5 |
CPT6 |
CPT7 |
CPT8 |
CPT9 |
1 |
Fixed object |
SV |
R |
TOT |
0.126 |
0.049 |
0.095 |
0.024 |
2 |
Animal |
SV |
R |
TOT |
0.021 |
0.009 |
0.000 |
0.005 |
3 |
Pedestrian |
SV |
R |
TOT |
0.001 |
0.003 |
0.010 |
0.011 |
4 |
Bicyclist |
SV |
R |
TOT |
0.003 |
0.005 |
0.010 |
0.009 |
5 |
Parked car |
SV |
R |
TOT |
0.006 |
0.006 |
0.000 |
0.005 |
6 |
Overturn |
SV |
R |
TOT |
0.060 |
0.027 |
0.029 |
0.007 |
7 |
Other single-vehicle |
SV |
R |
TOT |
0.068 |
0.100 |
0.019 |
0.068 |
8 |
Rear-End |
MV |
R |
TOT |
0.268 |
0.169 |
0.541 |
0.282 |
9 |
Head-On |
MV |
R |
TOT |
0.015 |
0.014 |
0.000 |
0.016 |
10 |
Angle |
MV |
R |
TOT |
0.299 |
0.472 |
0.229 |
0.422 |
11 |
Sideswipe, same direction |
MV |
R |
TOT |
0.055 |
0.060 |
0.019 |
0.053 |
12 |
Sideswipe, opposite direction |
MV |
R |
TOT |
0.014 |
0.013 |
0.010 |
0.010 |
13 |
Other multiple-vehicle |
MV |
R |
TOT |
0.064 |
0.073 |
0.038 |
0.088 |
1 |
Fixed object |
SV |
U |
TOT |
0.048 |
0.033 |
0.050 |
0.026 |
2 |
Animal |
SV |
U |
TOT |
0.017 |
0.008 |
0.012 |
0.002 |
3 |
Pedestrian |
SV |
U |
TOT |
0.011 |
0.009 |
0.008 |
0.013 |
4 |
Bicyclist |
SV |
U |
TOT |
0.010 |
0.010 |
0.014 |
0.013 |
5 |
Parked car |
SV |
U |
TOT |
0.003 |
0.002 |
0.001 |
0.002 |
6 |
Overturn |
SV |
U |
TOT |
0.012 |
0.008 |
0.011 |
0.005 |
7 |
Other single-vehicle |
SV |
U |
TOT |
0.043 |
0.074 |
0.021 |
0.043 |
8 |
Rear-End |
MV |
U |
TOT |
0.355 |
0.234 |
0.557 |
0.471 |
9 |
Head-On |
MV |
U |
TOT |
0.003 |
0.007 |
0.006 |
0.011 |
10 |
Angle |
MV |
U |
TOT |
0.375 |
0.483 |
0.241 |
0.300 |
11 |
Sideswipe, same direction |
MV |
U |
TOT |
0.039 |
0.041 |
0.026 |
0.041 |
12 |
Sideswipe, opposite direction |
MV |
U |
TOT |
0.006 |
0.007 |
0.003 |
0.005 |
13 |
Other multiple-vehicle |
MV |
U |
TOT |
0.078 |
0.084 |
0.050 |
0.068 |
1 |
Fixed object |
SV |
R |
FI |
0.126 |
0.049 |
0.095 |
0.024 |
2 |
Animal |
SV |
R |
FI |
0.021 |
0.009 |
0.000 |
0.005 |
3 |
Pedestrian |
SV |
R |
FI |
0.001 |
0.003 |
0.010 |
0.011 |
4 |
Bicyclist |
SV |
R |
FI |
0.003 |
0.005 |
0.010 |
0.009 |
5 |
Parked car |
SV |
R |
FI |
0.006 |
0.006 |
0.000 |
0.005 |
6 |
Overturn |
SV |
R |
FI |
0.060 |
0.027 |
0.029 |
0.007 |
7 |
Other single-vehicle |
SV |
R |
FI |
0.068 |
0.100 |
0.019 |
0.068 |
8 |
Rear-End |
MV |
R |
FI |
0.268 |
0.169 |
0.541 |
0.282 |
9 |
Head-On |
MV |
R |
FI |
0.015 |
0.014 |
0.000 |
0.016 |
10 |
Angle |
MV |
R |
FI |
0.299 |
0.472 |
0.229 |
0.422 |
11 |
Sideswipe, same direction |
MV |
R |
FI |
0.055 |
0.060 |
0.019 |
0.053 |
12 |
Sideswipe, opposite direction |
MV |
R |
FI |
0.014 |
0.013 |
0.010 |
0.010 |
13 |
Other multiple-vehicle |
MV |
R |
FI |
0.064 |
0.073 |
0.038 |
0.088 |
Table 37. Proportions of crashes by severity and type for crossroad ramp terminals and intersections. (Continued)
Crash No. |
Crash Type |
Number of Vehicles Involved |
Area Type |
Severity Level |
3ST |
4ST |
3SG |
4SG |
|
|
(SV, MV) |
(R, U) |
(TOT, FI) |
numeric |
numeric |
numeric |
numeric |
CPT1 |
CPT2 |
CPT3 |
CPT4 |
CPT5 |
CPT6 |
CPT7 |
CPT8 |
CPT9 |
1 |
Fixed object |
SV |
U |
FI |
0.048 |
0.033 |
0.050 |
0.026 |
2 |
Animal |
SV |
U |
FI |
0.017 |
0.008 |
0.012 |
0.002 |
3 |
Pedestrian |
SV |
U |
FI |
0.011 |
0.009 |
0.008 |
0.013 |
4 |
Bicyclist |
SV |
U |
FI |
0.010 |
0.010 |
0.014 |
0.013 |
5 |
Parked car |
SV |
U |
FI |
0.003 |
0.002 |
0.001 |
0.002 |
6 |
Overturn |
SV |
U |
FI |
0.012 |
0.008 |
0.011 |
0.005 |
7 |
Other single-vehicle |
SV |
U |
FI |
0.043 |
0.074 |
0.021 |
0.043 |
8 |
Rear-End |
MV |
U |
FI |
0.355 |
0.234 |
0.557 |
0.471 |
9 |
Head-On |
MV |
U |
FI |
0.003 |
0.007 |
0.006 |
0.011 |
10 |
Angle |
MV |
U |
FI |
0.375 |
0.483 |
0.241 |
0.300 |
11 |
Sideswipe, same direction |
MV |
U |
FI |
0.039 |
0.041 |
0.026 |
0.041 |
12 |
Sideswipe, opposite direction |
MV |
U |
FI |
0.006 |
0.007 |
0.003 |
0.005 |
13 |
Other multiple-vehicle |
MV |
U |
FI |
0.078 |
0.084 |
0.050 |
0.068 |
Table 38. Proportions of crashes by severity and type for arterial crossroad roadway segments.
Crash No. |
Crash Type |
Number of Vehicles Involved |
Area Type |
Severity Level |
1-Lane Undivided |
2-Lane Undivided |
3-Lane Undivided |
2-Lane Divided |
3-Lane Divided |
|
|
(SV, MV) |
(R, U) |
(TOT, FI) |
numeric |
numeric |
numeric |
numeric |
numeric |
CPC1 |
CPC2 |
CPC3 |
CPC4 |
CPC5 |
CPC6 |
CPC7 |
CPC8 |
CPC9 |
CPC10 |
1 |
Fixed object |
SV |
R |
TOT |
0.330 |
0.005 |
0.005 |
0.005 |
0.005 |
2 |
Animal |
SV |
R |
TOT |
0.377 |
0.061 |
0.061 |
0.120 |
0.120 |
3 |
Pedestrian |
SV |
R |
TOT |
0.001 |
0.004 |
0.004 |
0.004 |
0.004 |
4 |
Bicyclist |
SV |
R |
TOT |
0.000 |
0.000 |
0.000 |
0.001 |
0.001 |
5 |
Parked car |
SV |
R |
TOT |
0.000 |
0.000 |
0.000 |
0.001 |
0.001 |
6 |
Overturn |
SV |
R |
TOT |
0.023 |
0.004 |
0.004 |
0.005 |
0.005 |
7 |
Other single-vehicle |
SV |
R |
TOT |
0.043 |
0.161 |
0.161 |
0.296 |
0.296 |
8 |
Rear-End |
MV |
R |
TOT |
0.096 |
0.343 |
0.343 |
0.271 |
0.271 |
9 |
Head-On |
MV |
R |
TOT |
0.021 |
0.009 |
0.009 |
0.006 |
0.006 |
10 |
Angle |
MV |
R |
TOT |
0.021 |
0.185 |
0.185 |
0.089 |
0.089 |
11 |
Sideswipe, same direction |
MV |
R |
TOT |
0.014 |
0.039 |
0.039 |
0.043 |
0.043 |
12 |
Sideswipe, opposite direction |
MV |
R |
TOT |
0.026 |
0.011 |
0.011 |
0.005 |
0.005 |
13 |
Other multiple-vehicle |
MV |
R |
TOT |
0.048 |
0.178 |
0.178 |
0.154 |
0.154 |
1 |
Fixed object |
SV |
U |
TOT |
0.185 |
0.075 |
0.075 |
0.155 |
0.155 |
2 |
Animal |
SV |
U |
TOT |
0.173 |
0.003 |
0.003 |
0.129 |
0.129 |
3 |
Pedestrian |
SV |
U |
TOT |
0.002 |
0.019 |
0.019 |
0.001 |
0.001 |
4 |
Bicyclist |
SV |
U |
TOT |
0.000 |
0.013 |
0.013 |
0.000 |
0.000 |
5 |
Parked car |
SV |
U |
TOT |
0.000 |
0.013 |
0.013 |
0.000 |
0.000 |
6 |
Overturn |
SV |
U |
TOT |
0.007 |
0.010 |
0.010 |
0.013 |
0.013 |
7 |
Other single-vehicle |
SV |
U |
TOT |
0.031 |
0.004 |
0.004 |
0.043 |
0.043 |
8 |
Rear-End |
MV |
U |
TOT |
0.437 |
0.358 |
0.358 |
0.463 |
0.463 |
9 |
Head-On |
MV |
U |
TOT |
0.022 |
0.007 |
0.007 |
0.007 |
0.007 |
10 |
Angle |
MV |
U |
TOT |
0.046 |
0.359 |
0.359 |
0.056 |
0.056 |
11 |
Sideswipe, same direction |
MV |
U |
TOT |
0.027 |
0.102 |
0.102 |
0.075 |
0.075 |
12 |
Sideswipe, opposite direction |
MV |
U |
TOT |
0.023 |
0.012 |
0.012 |
0.015 |
0.015 |
13 |
Other multiple-vehicle |
MV |
U |
TOT |
0.047 |
0.025 |
0.025 |
0.043 |
0.043 |
1 |
Fixed object |
SV |
R |
FI |
0.330 |
0.005 |
0.005 |
0.005 |
0.005 |
2 |
Animal |
SV |
R |
FI |
0.377 |
0.061 |
0.061 |
0.120 |
0.120 |
3 |
Pedestrian |
SV |
R |
FI |
0.001 |
0.004 |
0.004 |
0.004 |
0.004 |
4 |
Bicyclist |
SV |
R |
FI |
0.000 |
0.000 |
0.000 |
0.001 |
0.001 |
5 |
Parked car |
SV |
R |
FI |
0.000 |
0.000 |
0.000 |
0.001 |
0.001 |
6 |
Overturn |
SV |
R |
FI |
0.023 |
0.004 |
0.004 |
0.005 |
0.005 |
7 |
Other single-vehicle |
SV |
R |
FI |
0.043 |
0.161 |
0.161 |
0.296 |
0.296 |
8 |
Rear-End |
MV |
R |
FI |
0.096 |
0.343 |
0.343 |
0.271 |
0.271 |
9 |
Head-On |
MV |
R |
FI |
0.021 |
0.009 |
0.009 |
0.006 |
0.006 |
10 |
Angle |
MV |
R |
FI |
0.021 |
0.185 |
0.185 |
0.089 |
0.089 |
11 |
Sideswipe, same direction |
MV |
R |
FI |
0.014 |
0.039 |
0.039 |
0.043 |
0.043 |
12 |
Sideswipe, opposite direction |
MV |
R |
FI |
0.026 |
0.011 |
0.011 |
0.005 |
0.005 |
13 |
Other multiple-vehicle |
MV |
R |
FI |
0.048 |
0.178 |
0.178 |
0.154 |
0.154 |
Table 38. Proportions of crashes by severity and type for arterial crossroad roadway segments. (Continued)
Crash No. |
Crash Type |
Number of Vehicles Involved |
Area Type |
Severity Level |
1-Lane Undivided |
2-Lane Undivided |
3-Lane Undivided |
2-Lane Divided |
3-Lane Divided |
|
|
(SV, MV) |
(R, U) |
(TOT, FI) |
numeric |
numeric |
numeric |
numeric |
numeric |
CPC1 |
CPC2 |
CPC3 |
CPC4 |
CPC5 |
CPC6 |
CPC7 |
CPC8 |
CPC9 |
CPC10 |
1 |
Fixed object |
SV |
U |
FI |
0.185 |
0.075 |
0.075 |
0.155 |
0.155 |
2 |
Animal |
SV |
U |
FI |
0.173 |
0.003 |
0.003 |
0.129 |
0.129 |
3 |
Pedestrian |
SV |
U |
FI |
0.002 |
0.019 |
0.019 |
0.001 |
0.001 |
4 |
Bicyclist |
SV |
U |
FI |
0.000 |
0.013 |
0.013 |
0.000 |
0.000 |
5 |
Parked car |
SV |
U |
FI |
0.000 |
0.013 |
0.013 |
0.000 |
0.000 |
6 |
Overturn |
SV |
U |
FI |
0.007 |
0.010 |
0.010 |
0.013 |
0.013 |
7 |
Other single-vehicle |
SV |
U |
FI |
0.031 |
0.004 |
0.004 |
0.043 |
0.043 |
8 |
Rear-End |
MV |
U |
FI |
0.437 |
0.358 |
0.358 |
0.463 |
0.463 |
9 |
Head-On |
MV |
U |
FI |
0.022 |
0.007 |
0.007 |
0.007 |
0.007 |
10 |
Angle |
MV |
U |
FI |
0.046 |
0.359 |
0.359 |
0.056 |
0.056 |
11 |
Sideswipe, same direction |
MV |
U |
FI |
0.027 |
0.102 |
0.102 |
0.075 |
0.075 |
12 |
Sideswipe, opposite direction |
MV |
U |
FI |
0.023 |
0.012 |
0.012 |
0.015 |
0.015 |
13 |
Other multiple-vehicle |
MV |
U |
FI |
0.047 |
0.025 |
0.025 |
0.043 |
0.043 |
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