Safety Implications of Managed Lane Cross Sectional Elements
CHAPTER 2: LITERATURE REVIEW
Managed lanes (ML) can provide safety and operational performance benefits over general-purpose (GP) facilities, but the managed lane strategy must be appropriate for the intended user group. Specific benefits in crash reduction seen at one facility do not necessarily translate to another facility, so the selected strategy must account for the conditions unique to a particular facility. This section presents a summary of recent freeway and managed lane safety research.
Crashes within the Managed Lane Facility
Crashes on managed lanes are assumed to most likely be related to access and sight distance issues. For some situations, a failure to appreciate driver expectancy that differs for managed lanes as compared to general-purpose lanes may contribute to crashes, for example, when drivers need to exit the managed lane several miles prior to their destination. Adequate attention to placement of traffic control devices can help.
In addition to crashes near access points, crashes can also occur within a managed lane facility. Common types of crashes within a facility can include:
Crashes at Access Points
Freeway access points are common sites for crashes, just as crashes can commonly be found at intersections on surface streets. Crashes near access points can involve vehicles entering or leaving the managed lanes (e.g., sideswipe, striking separation device, etc.), and crashes can involve vehicles that are not changing facilities (e.g., rear-end crashes caused by drivers braking to avoid a vehicle entering the facility in front of them). Traffic volumes, the type of access and separation provided, and proximity of managed lanes access to general-purpose entrance and exit ramps may all have an effect on crashes, and these effects may vary from one facility to another.
A California study described comparisons of traffic safety during the morning and afternoon peak hours in extended stretches of eight high occupancy vehicle (HOV) lanes with two different types of access – four corridors with continuous access and the others with limited access. (1, 2) Traffic crash patterns for the two different types of HOV lanes were investigated by evaluating (a) the differences in crash distribution, severity, types of crashes, and per lane traffic utilization; (b) spatial distribution of crash concentrations by using Continuous Risk Profile approach; and (c) crash rates in the vicinity of access points in HOV lanes with limited access. In their study, the researchers conducted detailed analysis on crash data during peak hours in relation to geometry and traffic features. Based on the findings from the assessment on eight routes, the limited-access HOV lanes appeared to offer no safety advantages over the continuous-access HOV lanes. Although the overall safety seemed comparable, the observed differences between these types of facilities were attributed to more frequent and concentrated distribution of crashes at limited-access HOV lanes.
A recent study examined two facilities in Minnesota: (3)
The authors used shockwave length as a surrogate of safety based on the assumption that the more vehicles involved in a slow-and-go maneuver, the higher the possibility a driver will fail to react in a timely manner. They commented that the source of traffic demands has a notable impact on performance with respect to safety and access. I-394 is operating well with the limited access because the majority of the demand is originating from three specific interchanges. In contrast, I-35W has a higher interchange density so the open-access philosophy is working well on that facility. The authors developed a software tool capable of defining the Optimal Lane Changing Regions (OLCRs) that can be used with planned HOT facilities that adopt a closed access philosophy. The proposed methodology defines OLCRs with respect to the positions of entrance or exit ramps. The second methodology was designed to support decisions for access restrictions on existing HOT facilities. The core is a developed model capable of emulating shockwave propagation on the HOT lane given target densities and speed differential between the HOT and the adjacent general-purpose lane.
Safety of Buffer-Separated High Occupancy Vehicle Lanes
A 2013 paper reported on an evaluation of the relationship between cross-section design (i.e., lane width, shoulder width, and buffer width) to safety performance for HOV lanes. (4) The authors used three years (2005 to 2007) of crash data for 13 southern California segments totaling 153 miles. The segments had the HOV lanes buffer-separated from the general-purpose lanes. Crashes included those that occurred on the median shoulder, in the HOV lane, or in the adjacent general-purpose left lane. Independent variables included geometric attributes and annual average daily traffic (AADT). The authors made the following observations regarding geometric cross section and crashes:
The authors stated that their findings could be used to determine the optimal cross-section and provided a case study discussion to illustrate the results. For one example, they recommended that a 12 ft lane and 10 ft left shoulder be converted to a 3.6 ft buffer, 12 ft lane, and 6.4 ft left shoulder. Two other examples were also provided in their paper, both of which suggested keeping the 12-ft lanes and shifting some of the left shoulder width into the buffer.
Cothron et al. and Cooner and Ranft reported on Texas research conducted to gain a better understanding of the safety issues and impacts associated with buffer-separated concurrent-flow HOV lanes. (5, 6) They reviewed hard-copy crash reports for multiple years from two urban freeways in Dallas, Texas. The objective was to determine trends and from those trends to make recommendations for absolute minimum and desirable buffer-separated concurrent flow HOV lane cross-section widths. In summary, researchers stipulate that the following factors all contribute to the increased injury crash rates experienced on the two Dallas corridors:
Figure 1 shows recommendations for desirable and absolute minimum cross sections for future buffer-separated HOV lanes in the Dallas area. The desirable cross-section guidance provides a typical section and a section with enforcement shoulders. Both desirable cross sections provide full inside shoulders and 4-ft buffers with a standard 12-ft lane for HOV traffic.
Source: adapted from Cooner, S., and S. Ranft. Safety Evaluation of Buffer-Separated High-Occupancy Vehicle Lanes in Texas. In Transportation Research Record: Journal of the Transportation Research Board, No. 1959, parts of the text, and Figure 8, p. 176. Copyright, National Academy of Sciences, Washington, D.C., 2006. Reproduced with permission of the Transportation Research Board. (6)
Note: abbreviations used in figure: GP = general purpose, HOV = high occupancy vehicle.
Cooner and Ranft provided the following summary of previous studies:(6)
Differences in Crashes between Given Conditions
An Empirical-Bayes statistical estimation procedure was conducted to evaluate the effects of tolling on the I-394 MnPass Lanes in Minnesota, which opened for operation in 2005. (14) AADT data were used from 1998 to 2008. A four-year observation period was used before the start of tolling as well as a two-year post deployment observation period. Crash data of interstate highways in the Minneapolis-St. Paul (Twin Cities) seven-county metropolitan area from the Minnesota Department of Transportation (DOT) were used. The study found the overall number of crashes to be reduced by 5.3 percent, with an economic benefit of $5 million from 2006 to 2008. The authors of the paper stated that they were not confident that their results could be transferred to other HOT lane projects because of the limited research on this issue, and the newness of many HOT lanes that have recently opened.
A 2012 paper presented results of a safety analysis of a time-of-day managed-lane strategy that concurrently allows use of the inner left lanes by high-occupancy vehicles and use of right shoulders as general-purpose lanes during peak hours. (15) The crash data (3 years), corresponding annual average daily traffic volumes, and lane-type-specific AADT volumes were identified for various lane types, including the inner left lanes for HOV-only use during peak hours, general-purpose lanes, right shoulder lanes, and all lanes as a whole. Negative binomial regression models were used to estimate the effect of this traffic operations system and other factors relevant to crash frequency. The negative binomial regression model analyses presented no evidence that the interest factors, including the managed-lane strategy during peak hours, AADT volumes, merging and diverging influence areas, weather, light conditions, and existence of pull-off areas affected the crash frequency when aggregated across all lanes. The variable AADT volumes in the specific analysis of general-purpose lanes appear to be significant and show about a 2 percent increase in weekday crashes for each increase of 1,000 vehicles per day in the AADT range of 50,000 to 83,000 vehicles per day. Right shoulder-specific analysis shows that motorist behaviors at the merge and diverge areas during adverse light conditions are significant and shows an increase of about 38 percent in crashes in these areas. The managed-lane strategy does not appear to be significant to the crash frequency in the inner left lanes for HOV, general-purpose lanes, or right shoulders.
The Highway Safety Manual (HSM) now includes crash predictive methods for freeways. (16) The developed chapters were based on research conducted as part of National Cooperative Highway Research Program (NCHRP) 17-45. (17) The researchers found that reductions in lane widths and inside (left) shoulder widths are associated with increased crashes. The proposed crash modification factor for the HSM along with the findings from other recent work is shown in Figure 2 for lane width and Figure 3 for inside (left) shoulder width. The range of shoulder widths included in the NCHRP 17-45 study was 2 to 12 ft. An inside shoulder width of 6 ft was assumed as the base condition. Some agencies avoid inside shoulder widths greater than 4 ft and less than 8 ft because of concerns that drivers may attempt to seek refuge in a space that does not have sufficient width to accommodate a typical vehicle (6 ft) plus clearance (desirably 1 ft to 2 ft as discussed in the Green Book, see Section 4.4.2, page 4-10). (7)
Freeway General-purpose Lane Cross Section
A recent Texas DOT project that examined the tradeoffs of reducing lane and shoulder widths to permit an additional freeway lane also identified increased crashes when the widths of lanes or shoulders are reduced. (18) The identified safety improvements included the following:
Reproduced with permission of the Transportation Research Board. (17)
Reproduced with permission of the Transportation Research Board.(17)
Source: Table 45 on page 76 from Dixon, K., K. Fitzpatrick, R. Avelar, M. Perez, S. Ranft, R. Stevens, S. Venglar, and T. Voigt (2015) Reducing Lane and Shoulder Width to Permit an Additional Lane on a Freeway: Technical Report. FHWA/TX-15/0-6811-1.
Source: Table 46 on page 77 from Dixon, K., K. Fitzpatrick, R. Avelar, M. Perez, S. Ranft, R. Stevens, S. Venglar, and T. Voigt (2015) Reducing Lane and Shoulder Width to Permit an Additional Lane on a Freeway: Technical Report. FHWA/TX-15/0-6811-1.
While the research also identified that an additional lane can result in reductions in crashes, whether the benefits of the additional lane completely offset the consequences of the reduced lane and shoulder widths would depend upon the conditions present at the site. The authors of the Texas study developed an equation and a spreadsheet that could be used to evaluate the tradeoffs. Note that the Texas work focused on freeways with general-purpose lanes rather than freeways that include a managed lane.
Freeways With High Occupancy Vehicle Or High Occupancy Toll Lanes
A Florida study developed crash prediction equations for freeways facilities with HOV and HOT lanes. (19) The authors developed unique models by number of freeway lanes. Models were developed for 6-, 8-, 10-, and 12-lane freeways (number of lanes reflect both directions and include the managed lanes). For all the models, segment length and AADT were significant and included. For most of the models, left shoulder width was the only other significant variable. An increase in left shoulder width was associated with decreases in crashes. The effect of buffer type on crashes was found to be statistically significant only in the model for 10-lane freeways with an inclusion of a 2- to 3-ft buffer being associated with fewer fatal and injury crashes. Figure 4 illustrates the findings for fatal+injury crashes and Figure 5 for all crashes.
Source: Figure 4.5 on page 40 from Srinivasan, S., P. Haas, P. Alluri, A. Gan, and J. Bonneson (2015) Crash Prediction Method for Freeway Facilities with High Occupancy Vehicle (HOV) and High Occupancy Toll (HOT) Lanes. FDOT Contract BDV32-977-04.
Note: abbreviations used in figure: FI = fatal and serious injury, AADT = annual average daily traffic.
Source: Figure 4.6 on page 40 from Srinivasan, S., P. Haas, P. Alluri, A. Gan, and J. Bonneson (2015) Crash Prediction Method for Freeway Facilities with High Occupancy Vehicle (HOV) and High Occupancy Toll (HOT) Lanes. FDOT Contract BDV32-977-04.
Note: abbreviation used in figure: AADT = annual average daily traffic.
United States Department of Transportation - Federal Highway Administration