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Publication Number: FHWA-RD-97-106

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 Upgrade
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