Rural Public Transportation Technologies: User Needs and Applications FR1-798
APPENDIX A
DEFINITIONS OF GEOMETRIC DESIGN
AND TRAFFIC VOLUME VARIABLES
This appendix presents definitions of the geometric design, traffic volume, and other related variables considered in the statistical modeling of interchange ramp and speed- change lane accidents. These definitions are presented in table 47. Table 47 includes both variables that were available in the existing Washington data base and variables that were derived from that data base and other highway agency records. The table identifies the variables that were considered in the analyses and whether each variable was applicable to ramp segments, entire
ramps, or speed-change lanes. The table also identifies whether each variable was continuous
or categorical in nature. Continuous variables are those with quantitative values on a
continuous scale. Categorical variables are those with a finite number of discrete levels or
categories. For each categorical variable, the tables in this appendix also identify the levels
or categories that were available for that variable. Not all levels were considered in the
statistical modeling; in some cases, because of sample size considerations, a particular category
had to be excluded from the analyses or merged with adjacent categories. This process is
described in section 5 of the main text of this report.
Table 47. Definitions of Variables Considered in Statistical Modeling of
Ramp and Speed-Change Lane Accidents
Variable |
Available for: |
Variable
type |
Levels for
categorical variables |
Ramp
segments |
Entire
ramps |
Speed-change
lanes |
Geometric Design Features |
Ramp type |
X |
X |
|
Categorical |
On-ramp
Off-ramp |
Speed-change lane type |
|
|
X |
Categorical |
Deceleration lane
Acceleration lane |
Ramp configuration |
X |
X |
X |
Categorical |
Diamond
Parclo loop
Outer connection
Direct connection
Semi-direct connection
Trumpet loop
Trumpet outer connection
Scissors connection
Buttonhook
Slip ramp to parallel one-
way frontage road
Collector/distributor (C/D)
road
Other ramp configuration |
Ramp segment type |
X |
|
|
Categorical |
Ramp proper
Two-way ramp proper
segment
Merge area on-ramp
Diverge area on-ramp
Weaving area on-ramp |
Speed-change lane type |
|
|
X |
Categorical |
Conventional deceleration
lane
Deceleration lane with
mainline lane drop
Conventional acceleration
lane
Acceleration lane with
mainline lane addition |
Number of lanes |
X |
|
X |
Categorical |
Range: 1 to 3 lanes |
Traveled-way width (ft) |
X |
|
X |
Continuous |
|
Average lane width (ft) |
X |
|
X |
Continuous |
|
Right shoulder width (ft) |
X |
|
X |
Continuous |
|
Left shoulder width (ft) |
X |
|
|
Continuous |
|
Minimum radius for any
horizontal curve (ft) |
|
X |
|
Continuous |
|
Horizontal alignment
index based on
equation (1) |
|
X |
|
Continuous |
|
Horizontal alignment
index based on
equation (1) with
expontent of Di term
equal to 1.0 |
|
X |
|
Continuous |
|
General grade of ramp |
|
X |
|
Categorical |
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