Right-turn lanes on state routes must be designed to accommodate semi-tractor trailer trucks as the design vehicle (WB-67). In many states, including Illinois, this results in a right-turn lane design with large turning radius and a channelized corner island, especially if the encroachment of trucks into other lanes is discouraged in the design. When the intersection angle is 90, this radius may place passenger vehicles at an excessive approach angle to the intersection. If the intersection is skewed, the approach angle for right-turning passenger vehicles may pose further challenges related to reduced sight distance. From 2006 to 2014, the Illinois Department of Transportation (IDOT) implemented modifications to ten right-turn approaches in the Peoria area. The changes were intended to improve the line of sight of right-turning passenger vehicles by reducing the skew of the approach angle, while allowing semi- tractor trailer trucks to make right turns without encroachment. Bradley University researchers were retained to conduct a two-part study; they (1) performed site-specific effectiveness evaluations of the modified sites, and (2) conducted an area-wide crash causation analysis of right-turn crashes. In the site-specific evaluation, driver behaviour analyses and traffic crash-based safety analyses were performed to assess the safety performance of the modified right-turn lane design. The results of the crash causation analysis were used to identify the geometric variables that influence right-turn crashes to make recommendations on what types of sites (based on their characteristics) would make good candidates for the installation of the modified right-turn lane design in Illinois. Field observations of driver behaviour at right-turn lanes at ten test sites and ten control sites were collected to assess the modified right-turn lane design. The test and control comparisons revealed that drivers travelling through the modified right-turn lane design (test sites) used safer driving behaviours compared with the control sites with the traditional design. Drivers at the test sites: * Used fewer exaggerated head turns - improved line of sight when checking for upstream traffic to the left * Used fewer roll-and-go stops - minimized opportunity for traffic conflicts * Stopped on or before the stop bar more frequently - drivers are able to stop at the proper location with adequate sight distance Because drivers at the modified right-turn lane design use fewer exaggerated head turns, roll-and-go stops, and stops past the stop bar, they will essentially travel at slower speeds, have an improved line of sight, and approach the right-turn with less skew. The traffic crash–based evaluation of seven test approaches with three years of before- and three years of after-modification data also provides insight into the safety impact of the modified right-turn lane design. The results of the empirical Bayes (EB) method indicated statistically significant reductions in subject approach crashes (59.0%), and right-turn crashes at the subject approaches (59.6%) at 95% level of confidence (LOC). Additional analyses were conducted to determine the impact of the modified design on two subsets of drivers, older (age 65+) and younger (age 16 to 21 years). The findings from the older- and younger-driver analyses were based on the naïve before and after method because the needed safety performance functions (SPFs) for the EB method are not available for these age categories. The following are the statistically significant reductions observed: * Older-driver right-turn crashes at the subject approach were significantly reduced by 70% * Younger-driver crash analysis revealed significant reductions of 43% for intersection crashes, 63% for approach crashes, and 66% for right-turn crashes at the subject approach Using the procedures outlined in the Highway Safety Manual (AASHTO 2010), crash modification factors (CMFs) were developed for each crash type. The resulting CMFs, along with their confidence intervals (CI), are as follows: * Subject approach crashes CMF = 0.410 with 95% CI = 0.224 to 0.596 * Right-turn crashes at the subject approach CMF = 0.404 with 95% CI = 0.183 to 0.625 Overall, the safety evaluation provides evidence that modifying right-turn lanes to sharpen the approach angle of right-turn vehicles improves safety by reducing the frequency of right-turn crashes at the approaches, as studied in the Peoria, Illinois, area. An analysis was conducted to determine the economic effectiveness of the modified right-turn lane design at seven test sites in the Peoria area. Economic costs and benefits of the modified design were calculated and annualised to determine the benefit to cost ratio of the modified right-turn lane design. The resulting benefit to cost ratio indicates that the accrued benefits in dollar value exceeds the annualised cost of the modified right-turn lane design over a period of 15 years by a factor of 13.8. Based on the overall finding of the study, both the driver behaviour and safety evaluations, the modification of right-turn lanes to improve the approach angle of right-turning vehicles has significant benefits. It is recommended that similar modifications to improve the right-turn approach angle/position be considered as potential improvements for intersection approaches that experience a pattern of right-turn crashes. In total, 116 right-turn approaches located throughout Illinois, with 3,174 right-turn crashes over a four-year period were included in the crash causation analysis. Crash rates for right-turn crashes and injury crashes were calculated using the traditional definition of crash rate (crashes per million entering vehicles) and an alternate one (crashes per million entering right-turn approach and intersecting approach vehicles). This resulted in four crash rates, with two rates for right-turn crashes and two for right-turn injury crashes for each of the 116 study approaches. Once the crash and injury rates were calculated, statistical analyses were performed to determine whether there was a significant difference in crashes among a series of two or three comparison groups for the following variables at 95% LOC: * Right-turn approach location (acute quadrant vs. 90 intersection angle vs. obtuse quadrant) *Intersection angle (intersection angle - 75 vs. intersection angle < 75) * Right-turn angle (right-turn angle - 45 vs. right-turn angle < 45) * Head-turn angle (head-turn angle - 140 vs. head-turn angle > 140) * Length of right-turn radius (radius - 100 feet vs. radius > 100 feet) * Channelized right-turn approach (channelized vs. unchannelized) Based on the analysis of right-turn crashes, two characteristics were found to be significant from these comparisons. Approaches with right-turn angles less than 45 and head-turn angles greater than 140 were associated with significantly higher crash rates. Based on the analysis of right-turn injury crashes, three of the geometric variables were found to be significant. Right-turn injury crash rates were significantly higher for approaches with intersection angles less than 75, right-turn angles less than 45, and head-turn angles greater than 140. Correlations between right-turn crashes and the geometric characteristics and traffic volume data were investigated through regression analyses using a negative binomial underlying distribution. It was found that head-turn angle and right-turn radius had a statistically significant impact on right- turn crashes. Based on the findings of the effectiveness evaluation and the crash causation study, flexibility in design may be needed for intersections and right-turn approaches when one or more of the following characteristics exist because higher right-turn crash rates may result: * Channelized island present with large right-turn radius Intersection angle < 75 * Right-turn angle 25 to 30, or < 45 * Head-turn angle > 140 * Right-turning volume > 250 vph, or right-turn approach ADT > 3,125 vpd * Moderate truck volumes * Five or more right-turn crashes at an approach per year Intersections and right-turn approaches that exhibit a combination of these characteristics may experience higher right-turn crash rates. The modified right-turn lane design should be considered as a safety countermeasure at approaches with these noted characteristics. (Author/publisher)
Samenvatting