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Publication Number: FHWA-RD-03-037
Date: May 2005

Validation of Accident Models for Intersections

FHWA Contact: John Doremi,
HRDI-10, (202) 493-3052, John.doremi@dot.gov

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1. INTRODUCTION

Effective safety management of a highway system requires that engineers know the present safety performance of a roadway and how it will perform if contemplated actions are taken. In effect, a reliable method of estimating safety performance is required. To this end, FHWA and its contractors have developed a new approach that combines historical accident data, regression analysis, before-and-after studies, and expert judgment to make safety performance predictions that are expected to be better than those obtained by any of the individual approaches. A recent report documents an accident prediction algorithm for implementing the new approach for two-lane rural highway sections that include road segments and five types of intersections.(3) Ongoing efforts aim to produce similar documents for other types of facilities.

The accident prediction algorithm has been developed for incorporation in the IHSDM as the Crash Prediction Module, but is suitable for stand-alone applications. The structure of the accident prediction algorithm for the five types of rural at-grade intersections is as follows:

Equation 1.  The predicted number of total intersection-related accidents per year after application of AMFs, N subscript INT, equals the predicted number of total intersection-related accidents per year for base conditions, N subscript B, times the AMF for various intersection features, AMF subscript 1, AMF subscript 2, continuing through AMF subscript N, where N is the last intersection feature. (1)

where

Nint = predicted number of total intersection-related accidents per year after application of AMFs;

Nb = predicted number of total intersection-related accidents per year for base conditions; and

AMF1 AMF2 ... AMFn = AMFs for various intersection features.

Harwood et al. presented base models and AMFs for three- and four-legged intersections of two-lane rural roads with STOP control, and four-legged signalized intersections of two-lane roads.(3) These base models were the best of available accident prediction models developed in earlier FHWA projects and retained model variables that were statistically significant at the 15 percent level.(1,2) Those projects also resulted in regression models with additional variables indented for use as AMFs in IHSDM. The full models, along with several variants, are presented in two FHWA reports: Vogt and Bared present models for three- and four-legged intersections of two-lane roads, while Vogt documents models for three other types of rural intersections: three- and four-legged stop controlled with four lanes on the major and two on the minor; and signalized intersections of two-lane roads. (1,2) In summary, there are five types of intersection accident prediction models pertaining to the research efforts described:

  • Type I: Three-legged stop controlled intersections of two-lane roads.
  • Type II: Four-legged stop controlled intersections of two-lane roads.
  • Type III: Three-legged stop controlled intersections with two lanes on the minor and four lanes on the major road.
  • Type IV: Four-legged stop controlled intersections with two lanes on the minor and four lanes on the major road.
  • Type V: Signalized intersections of two-lane roads.

The models were developed using data that were limited in terms of geographical diversity and, in the case of Types III, IV, and V, sample size. Thus, validation of these models across both space and time has become of paramount importance. It was also of interest to validate the accident prediction algorithm as a whole, given its novelty and the fact that it relies on expert judgment for deriving the AMFs. An additional report provides additional AMFs for left- and right-turn lanes for at-grade intersections at type I, II and V sites.(5) These AMFs are also included in the validation effort.

A natural follow-on to model validation is model recalibration using validation data and findings to improve the specification of the intersection models. This report presents recalibration results for the five types of rural intersections that were the subject of the validation exercise that was undertaken in the first part of the project. This model recalibration effort complemented the comprehensive model validation conducted as part of a larger technical evaluation of crash prediction models. AADT models and fully parameterized models were recalibrated, and their results are discussed in the recalibration chapter.

This report consists of three chapters. This first chapter provides an introduction and presents the description of the variables used in this research. The second chapter describes four different sets of activities conducted to assess the validity of prediction models for the five types of rural intersections. The third chapter presents the recalibration efforts for the five types of intersections. The model recalibration efforts described in this chapter complement the comprehensive model validation exercise presented in the previous chapter as a part of larger technical evaluation of the models.

1.1 VARIABLE ABBREVIATIONS

This section provides, for ease of reference, the definitions of model types, accident types, and variables applied in this research investigation.

1.1.1 Model Types

  • Type I: Three-legged stop controlled intersections of two-lane roads.
  • Type II: Four-legged stop controlled intersections of two-lane roads.
  • Type III: Three-legged stop controlled intersections with two lanes on the minor and four lanes on the major road.
  • Type IV: Four-legged stop controlled intersections with two lanes on the minor and four lanes on the major road.
  • Type V: Signalized intersections of two-lane roads.

1.1.2 Accident Models

Models I and II

Total: Total number of police-reported intersection-related accidents within 76.25 meters (m) (76.25 m (250 ft) (ft)) of the intersection.

Injury: Total number of police-reported intersection-related injury accidents within 76.25 m (250 ft) of the intersection.

Models III, IV, and V

TOTACC: Total number of accidents within 76.25 m (250 ft) of the intersection.

TOTACCI: Only those crashes considered intersection-related and within 76.25 m (250 ft) of the intersection.

INJACC: Total number of injury crashes within 76.25 m (250 ft) of the intersection.

INJACCI: Only those injury crashes considered intersection-related and within 76.25 m (250 ft) of the intersection.

1.1.3 Definitions of Variables

AADT1: Average daily traffic on major road (vehicles per day). This variable is identical to ADT1 in the original (published) models. This change was made after determining that the traffic flow variables were, in fact, estimated AADT.

AADT2: Average daily traffic on minor roads (vehicles per day). This variable is identical to ADT2 in the original models.

COMDRWY1: Commercial driveways on major roads within 76.25 m (250 ft) of the intersection center. This variable is identical to NODRWYC1 in the original models.

COMDRWY2: Commercial driveways on minor roads within 76.25 m (250 ft) of the intersection center. This variable is identical to NODRWYC2 in the original models.

DRWY1: Driveways on major roads within 76.25 m (250 ft) of the intersection. This variable is identical to ND and NODRWY1 in the original models for Type I-II and III-V intersections, respectively.

DRWY2: Driveways on minor roads within 76.25 m (250 ft) of the intersection. This variable is identical to NODRWY2 in the original models.

GRADE1: Average absolute grade on major roads within ±244 m (±800 ft) of the intersection center (percent).

GRADE2: Average absolute grade on minor road within ±244 m (±800 ft) of the intersection center (percent).

HAU: Intersection angle variable defined where the angle between the major and minor roads is measured from the far side of the minor road:

  • Three-legged intersections: Angle minus 90 if minor road is to the right of the major road in the increasing direction; 90 minus angle if minor road is to the left of the major road in the increasing direction.
  • Four-legged intersections: (right angle - left angle)/2.

HAZRAT1: Roadside hazard rating on major road within 76.25 m (250 ft) of the intersection center (from 1, least hazardous case, to 7, most hazardous case).(6) This variable is identical to RHRI in the original models for Type I and II intersections.

HAZRAT2: Roadside hazard rating on minor road within 76.25 m (250 ft) of the intersection center (from 1, least hazardous case, to 7, most hazardous case).(6)

HEI1: Sum of degree of curve in degrees per hundred feet of each horizontal curve on major road, any portion of which is within ±244 m (±800 ft) of the intersection center divided by the number of such curves.

HEI2: Sum of degree of curve in degrees per hundred feet of each horizontal curve on minor road, any portion of which is within ±244 m (±800 ft) of the intersection center divided by the number of such curves.

HEICOM: (1 / 2) (HEI1 + HEI2).

HI: Sum of degree of curve in degrees per hundred feet of each horizontal curve on major road, any portion of which is within 76.25 m (250 ft) of the intersection center divided by the number of such curves.

HI1: Sum of degree of curve in degrees per hundred feet of each horizontal curve on major road, any portion of which is within 76.25 m (250 ft) of the intersection center divided by the number of such curves. This variable is identical to HI in the original models for Type I and II intersections.

HI2: Sum of degree of curve in degrees per hundred feet of each horizontal curve on minor road, any portion of which is within 76.25 m (250 ft) of the intersection center divided by the number of such curves.

HICOM: (1 / 2) (HI1 + HI2).

L1LT (Type III-V): Left-turn lane on major roads (0 = no, 1 for one approach, and 2 for both approaches). This variable is identical to LTLN1 in the original models for Type III-V intersections.

L1RT (Type III-V): Right-turn lane on major roads (0 = no, 1 for one approach, and 2 for both approaches). This variable is identical to RTLN1 in the original models for Type III-V intersections.

L3LT (Type III-V): Left-turn lane on minor roads (0 = no, 1 for one approach, and 2 for both approaches). This variable is identical to LTLN2 in the original models for Type III-V intersections.

L3RT (Type III-V): Right-turn lane on minor roads (0 = no, 1 for one approach, and 2 for both approaches). This variable is identical to RTLN2 in the original models for Type III-V intersections.

LEGACC1: Acceleration lane on major roads (0 = no, 1 = yes).

LEGACC2: Acceleration lane on minor roads (0 = no, 1 = yes).

LIGHT: Light at intersection (0 = no, 1 = yes).

LTLN1S (Type III-V): Left-turn lane on major roads (0 = no, 1 = yes).

LT MAJ (Type I-II): 1 if left-turn lane exists on at least one approach of major roads, 0 otherwise.

LT MIN (Type I-II): 1 if left-turn lane exists on at least one approach of minor roads, 0 otherwise.

MEDIAN (Type I-II): 1 if median exists on major roads, 0 otherwise.

MEDTYPE (Type III-V): Median type (0 = no median, 1 = painted, 2 = curbed, 3 = others).

MEDWDTH1: Median width on major roads (feet). This variable is identical to MEDWIDTH1 in the original models.

MEDWDTH2: Median width on minor roads (feet).

PKLEFT: Peak left-turn percentage (percent).

PKLEFT1: Peak left-turn percentage on major roads (percent).

PKLEFT2: Peak left-turn percentage on minor roads (percent).

PKTHRU1: Peak through percentage on major roads (percent).

PKTHRU2: Peak through percentage on minor roads (percent).

PKTRUCK: Peak truck percentage passing through the intersection (percent).

PKTURN: Peak turning percentage (percent).

PROT_LT: Protected left lane (0 = no, 1 = yes).

RESDRWY1: Residential driveways on major roads within 76.25 m (250 ft) of the intersection center. This variable is identical to NODRWYR1 in the original models.

RESDRWY2: Residential driveways on minor roads within 76.25 m (250 ft) of the intersection center. This variable is identical to NODRWYR2 in the original models.

RT MAJ (Type I-II): 1 if right-turn lane exists on major roads, 0 otherwise.

RT MIN (Type I-II): 1 if right-turn lane exists on minor roads, 0 otherwise.

SD1: Longitudinal sight distance on major roads (feet).

SDL2: Left-side sight distance on minor roads (feet).

SDR2: Right-side sight distance on minor roads (feet).

SHOULDER1: Shoulder width on major roads (feet).

SHOULDER2: Shoulder width on minor roads (feet).

SPD1: The average posted speed on major roads in vicinity of the intersection (miles per hour). This variable is identical to SPDI in the original models for Type I and II intersections.

SPD2: The average posted speed on minor roads in vicinity of the intersection (miles per hour).

TERRAIN (Type I-II): 1 if flat, 2 if rolling, or 3 if mountainous terrain.

TERRAIN1 (Type III-V): Terrain on major roads within 76.25 m (250 ft) of the intersection center (0 = flat, 1 = rolling, 2 = mountainous).

TERRAIN2 (Type III-V): Terrain on minor roads within 76.25 m (250 ft) of the intersection center (0 = flat, 1 = rolling, 2 = mountainous).

VCEI1: Sum of absolute change of grade in percent per hundred feet for each crest curve on major roads, any portion of which is within 800 feet of the intersection center, divided by the number of such curves.

VCEI2: Sum of absolute change of grade in percent per hundred feet for each crest curve on minor roads, any portion of which is within 800 feet of the intersection center, divided by the number of such curves.

VCI1: Sum of absolute change of grade in percent per hundred feet for each crest curve on major roads, any portion of which is within 76.25 m (250 ft) of the intersection center, divided by the number of such curves. This variable is identical to VCI in the original models for Type I and II intersections.

VCI2: Sum of absolute change of grade in percent per hundred feet for each crest curve on minor roads, any portion of which is within 76.25 m (250 ft) of the intersection center, divided by the number of such curves.

VEI1: Sum of absolute change of grade in percent per hundred feet for each curve on major roads, any portion of which is within ±244 m (±800 ft) of the intersection center, divided by the number of such curves.

VEI2: Sum of absolute change of grade in percent per hundred feet for each curve on minor roads, any portion of which is within ±244 m (±800 ft) of the intersection center, divided by the number of such curves.

VEICOM: (1 / 2) (VEI1 + VEI2).

VI1: Sum of absolute change of grade in percent per hundred feet for each curve on major roads, any portion of which is within 76.25 m (250 ft) of the intersection center, divided by the number of such curves.

VI2: Sum of absolute change of grade in percent per hundred feet for each curve on minor roads, any portion of which is within 76.25 m (250 ft) of the intersection center, divided by the number of such curves.

VICOM (Type III-V): (1 / 2) (VI1 + VI2).

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