Safety Prediction Methodology and Analysis Tool for Freeways and Interchanges (2021)

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349 CHAPTER 9: CONCLUSIONS AND RECOMMENDATIONS The following conclusions have been developed based on the research conducted for this project. ● This report documents a safety prediction method for. The method addresses freeway segments and freeway speed-change lanes. It includes CMFs that describe the observed relationship between crash frequency and horizontal curvature, lane width, shoulder width, median width, barrier length and offset, ramp-related lane changes, rumble strip presence, clear zone width, and the extent of recurring congestion. ● This report documents a safety prediction method for ramps. The method addresses ramp segments, C-D road segments, and crossroad ramp terminals. For segments, it includes CMFs that describe the observed relationship between crash frequency and horizontal curvature, lane width, shoulder width, barrier length and offset, a change in the number of basic lanes, presence of a ramp-to-ramp merge or diverge point, and ramp-related lane changes on a C-D road. For crossroad ramp terminals, the method includes CMFs that describe the observed relationship between crash frequency and exit ramp control, exit ramp lanes, presence of turn lanes on the crossroad, presence of driveway access points, distance to the adjacent ramp terminal, median width, presence of protected-only left-turn operation, presence of right-turn channelization, and skew angle. ● The aforementioned safety prediction methods use a disaggregate approach for evaluating freeway sections, ramps, C-D roads, or entire freeway facilities within the limits of a project. The roadway of interest is initially separated into individual segments and intersections. Each segment has similar geometry and traffic conditions along its length. Each segment and intersection is then evaluated using the prediction method. The results for each segment and intersection are then combined to yield safety information for the overall project. For each segment or intersection, the method can be used to produce an estimate of the expected crash frequency by crash severity, crash type, or both. ● The safety prediction method was developed using data for California, Maine, and Washington. It includes a procedure for calibrating the models to local conditions. The data from the state of Maine was used to validate the models after they were calibrated using data for the other two states. The following recommendations have been developed based on the research conducted for this project. ● ● The safety prediction method for freeways is believed to be sufficiently complete that it can be used to address all rural freeways and most urban freeways. However, its coverage

350 could be broadened through the conduct of future research to incorporate models that quantify the safety effect of the following geometric design and traffic control features. ○ Freeways with 11 or more through lanes in urban areas. ○ Freeways with 9 or more through lanes in rural areas. ○ Freeways with continuous access HOV lanes. ○ Freeways with limited access managed lanes that are buffer-separated from the general-purpose lanes. ○ Freeways with ramp metering. ○ Use of safety shoulders as travel lanes. ● The analysis of freeway crash data indicated that crashes on curved freeway segments with shoulder rumble strips were more frequent than on curved segments without shoulder rumble strips. This finding is partially supported by other research documented in the literature. Additional research is needed to quantify the safety effect of shoulder rumble strips on freeway curves. ● The safety prediction method for ramps is believed to be sufficiently complete that it can be used to address all rural ramps and most urban ramps. However, its coverage could be broadened through the conduct of future research to incorporate models that quantify the safety effect of the following geometric design and traffic control features. ○ Ramp or C-D road segments in rural areas with 2 or more lanes. ○ Ramp or C-D road segments in urban areas with 3 or more lanes. ○ Ramps and C-D roads providing two-way travel. ○ Ramps with ramp metering. ○ A HOV bypass lane on a ramp or C-D road. ○ A crossroad speed-change lane. ○ A crossroad ramp terminal with 3 or more left-turn lanes on a crossroad approach. ○ A crossroad ramp terminal where the crossroad provides one-way travel. ○ The crossroad ramp terminal formed by a SPUIs or roundabout. ● The safety prediction method for ramps does not address frontage roads. Frontage roads are sufficiently unique in their design and operation that a separate safety prediction method should be developed to specifically address them. This method would include predictive models that separately address one-way frontage road segments, two-way frontage road segments, frontage road ramp terminals, and frontage road crossroad terminals. ● An interim predictive method for all-way stop control is included in the safety predictive method for crossroad ramp terminals. It is based on the use of an adjustment factor with the predictive model for one-way stop-controlled terminals. Additional research is needed to develop a more robust predictive model for terminals with this control mode.