Evaluation of Geometric Design Needs of Freeway Systems Based on Safety and Geometric Data
Freeways are arterial highways characterized by high levels of safety and high speed vehicular traffic. Access to and from the freeways is provided through ramps. Geometric elements making up freeway facilities include the roadway, median shoulders, grades, and ramps to and from the traveled way at selected locations, shoulders, radius of curvature, lane width, and speed-change lanes. With the increase of traffic using the freeway systems, there arises more traffic weaving movements within the elements making up the freeway systems. This causes traffic flow to compete at the limited spaces available and reduces safety performance of freeway system.
In studies on safety issues of freeway systems, geometric elements of freeways have been evaluated for their safety effects on crashes occurring on the freeways. These studies have included interchange spacing, number of through lanes, median shoulder width and type, ramp spacing, length of segment, speed change lanes, and lengths for limited and extended lanes. Their findings revealed that freeway safety issues are associated with freeway geometric characteristics. However, the previous studies did not consider the safety impact of all segment types on the crash frequency on freeways. This study observed four types of segments when a freeway is divided into segments with Exit and Entry terminals. These segments were defined as EN-EN, EX-EX, EN-EX, EX-EN segments where "EX" stands for Exit from the freeway and "EN" stands for Entrance to the freeway. The study also extends types of weaving movements taking place in weaving segments.
Crash rate and severity models were developed in this study based on the data collected for every freeway segment type. A complete set of geometric data was included in the data for each freeway segment type. Models for individual freeway segment type (EN-EN, EX-EX, EN-EX, and EX-EN) were developed. The results indicated that for EN-EN segment type; only two freeway characteristics had an impact: median width and segment length. Wider median and long segments both reduced crash while they were insignificant for severity model.
For EX-EX segment type, the number of through lanes, median width, and AADT had an impact on average crash rate while for a severity model, only the number of through lanes had an impact. Specifically, it was found that, the number of through lanes reduced both average crash rate and high severity crashes when all through lanes were combined together. However, on individual segment type in a specific freeway, it was found that, the number of through lanes on I-15 increased average crash rate while they reduced average crash rate on I-215. Wider median reduced average crash rate while it increased high severity crashes. Traffic volume increased average crash rate while it was found insignificant on severity model. At a freeway level, EX-EX segment type reduced average crash rate compared to both I-215 and US95 while it reduced average crash rate for I-215 compared to I-15 and US95.
For EN-EX segment type, shoulder width had a significant impact on average crash rates while the number of through lanes, median width, length of segment, and curve radius indicated significant impact on severity crashes. Wider shoulders on I-15 reduced average crash rate. The number of through lanes increased high severity crashes when all number of lanes were combined together. However, on individual freeways, the number of through lanes on reduced high severity crashes while they were insignificant on I-215 and US95. Wider median increased high severity crashes when all freeways were combined together while they reduced high severity crashes on I-15. Long segment increased high severity crashes when all EN-EX segment type from all freeways was combined together. Segments with large radius of curvature reduced high severity crashes when all for combined freeways while they increased high severity crashes for
I-15. At a freeway level, I-15 increased both average crash rate and high severity crashes compared to I-215 and US95.
For EX-EN segments, shoulder and AADT had a significant impact on average crash rate while the number of through lanes, median width, radius curvature and lane changes from ramp-to-freeway had a significant impact on severity crashes. Wider shoulder reduced average crash rate for combined data from all freeways but increased crash rate on I-215.Wider median increased high severity crashes for combined data from all freeways while they were insignificant on average crash rate models. Segments with large radius of curvature increased high severity crashes while it was insignificant on average crash rate model. Lane changes from ramp-to-freeway increased high severity crashes. AADT increased average crash rate while it was found insignificant on severity crashes.