Safety Implications of Managed Lane Cross Sectional Elements
CHAPTER 6: SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS FOR FUTURE RESEARCH
The managed lanes included in this study are designated freeway lanes separated by a buffer and located to the left of the general-purpose lanes. Managed lanes are intended to provide faster travel speeds and better reliability than the adjacent general-purpose lanes. Questions are being asked on whether the improved operations and the potentially additional buffer separation distances are associated with more or less crashes. This study investigated the relationship between crashes and buffer-separated manage lane dimensions.
The findings from the safety literature are clear in that reduction in a freeway left shoulder width is associated with increased number of crashes (see, for example, Figure 3). Safety studies for general-purpose freeway lanes also have found that reduction in lane width is associated with more crashes (see, for example, Figure 2). Previous research has provided the following safety relationships for freeways and managed lanes:
To better focus this research so to improve the likelihood of identifying usable sites that fit the objective of this project, data collection focused on sites with one (rather than two) managed lane(s) per direction that were operational 24 hours a day, seven days a week. Sites that represented a range of buffer widths with and without pylons were also sought. For this study only those segments that were non-weaving segments were included in the analysis.
The datasets used in this evaluation included 128.0 miles in California (all 128.0 miles were sections with flush buffers) and 60.4 miles in Texas (40.7 miles with pylon buffers and 18.7 miles with flush buffers). The California crash data included the years 2007 through 2011 while the years 2009 to 2014 were used for Texas crash data. The analysis was conducted on non-weaving managed lane segments that included a single managed lane separated from the general purpose lanes with a flush buffer area. The dataset included crashes on 128.0 miles in California (all 128.0 miles with flush buffers) and 60.4 miles in Texas (41.7 miles with pylon buffers and 18.7 miles with flush buffers). The California sites included freeways with three or four general-purpose lanes while the Texas freeways had three to five general-purpose lanes.
All of the buffers for the California segments were flush (i.e., no pylons) with widths that varied between 1 ft and 12 ft. The buffers generally consisted of white and yellow lane line markings. The larger buffer widths (9 or 12 ft) were associated with preserving space for a downstream managed lane ramp. The buffers in Texas include flush buffers and flush buffers with pylons. The Texas sites with flush buffers ranged between 1.5 and 5.0 ft, while the buffers with pylons were between 4.0 and 6.0 ft.
The analysis of the Texas and California data showed the following:
Trends in the data clearly suggest that fewer crashes are associated with wider buffer widths. However, an attempt to quantify this trend resulted in strongly correlated estimates.
When exploring whether a particular component of the managed lane envelope is more influential than another, the simultaneous evaluation on the three envelope components using California freeway crashes (all severity levels) identified the left shoulder width as statistically significant. Another modeling technique of using the proportion of California managed-lane related crashes to general-purpose crashes revealed that given a crash occurred, the odds of a crash being a managed-lane related crash:
An evaluation was also performed on the crashes that were coded as being on the managed lane or on the buffer. The refined model on managed lane and buffer-related crashes in California that only includes the significant variables found that all three components of the managed lane envelope – left shoulder width, lane width, and buffer width – are significant along with the volume in the managed lane. The results indicate that the lane width is the most influential followed by the buffer width. Changes in left shoulder width are not as influential in the number of managed-lane or buffer related crashes as the buffer or lane width changes.
In summary, the key findings from this study include the following:
Recommendations for Future Research
Benefits of Pylons
Freeway cross sections used to accommodate managed lanes vary. At some locations, managed lanes are separated from general-purpose freeway lanes using an exclusive alignment or using barriers. Other locations use a buffer where the buffer consists of a flush area marked with pavement markings and in some cases with supplemental pylons. The findings from this research are that wider managed lane envelopes are associated with fewer crashes and pylons appear to be associated with more crashes. The dataset used to identify the pylon crash relationship presented two important challenges: dataset size and use of freeway (rather than managed lane) crashes. If only those crashes near the pylons are considered (say for the lanes on either side of the pylons) different results may be present. The available data for this study were limited in total number of miles and only one state had data with pylons. The benefits (or disadvantages) of using pylons (and the available buffer width present between the travel lane and the pylon) need additional investigations.
Safety Tradeoffs when Adding a Managed Lane to an Existing Freeway
More crashes are associated with freeway lane and shoulder widths reductions. The increase in crashes may be offset if the reductions are done to increase the number of freeway lanes. A Texas study (18) developed a methodology and spreadsheet that can be used to evaluate the tradeoffs; however, the presence of managed lanes was not included in that research. Research is needed to determine the tradeoffs when the added lane is a managed lane.
Safety Differences between Managed Lanes with One Lane and with More than One Lane
This study focused on sites where only one managed lane was present per direction of travel. It is unclear how safety of these sites would compare to sites where additional managed lanes per direction are present. Because of the limited space in urban environments, the decision of adding managed lanes per direction may come at the cost of eliminating general-purpose lanes. The tradeoffs of potential cross-section changes would benefit from research.
Safety / Operations Differences Considering Congestion Levels
Improved level of service and reliability are two important attractors for potential users of managed lanes. Freeway operations and safety are expected to change as more users begin to utilize managed lanes, as this pattern is probably coupled with a decrease of traffic and congestion in the general-purpose lanes. It is unknown if such re-balance of the traffic demand and congestion translates into a measurable safety shift.
United States Department of Transportation - Federal Highway Administration