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 |
Leverls for categorical variables |
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| Ramp segments |
Entire ramps |
Speed-change lanes |
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| Geometric Design Features | ||||||
| Ramp type | X | X | Categorical | On-ramp Off-ramp | ||
| Speed-change lane type | X | Categorical | Deceleration lane Acceleration lane |
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| 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 parrellel 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 |
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| Speed-change lane type | X | Categorical | Conventional deceleration lane Deceleration lane with : : mainline lane drop Conventional acceleration lane Acceleration lane with mainline lane addition |
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| 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 Downgrade |
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