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Publication Number: FHWA-HRT-09-061
Date: February 2010
Simulator Evaluation of Low-Cost Safety Improvements on Rural Two-Lane Undivided Roads: Nighttime Delineation for Curves and Traffic Calming for Small Towns
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This report describes a driving simulator experiment designed to evaluate two sets of alternative low-cost safety improvements for rural areas. The experiment was sponsored by the Low Cost Safety Improvements Pooled Fund Study. The first set of improvements was directed toward enhancing the visibility of curves on rural two-lane undivided roads at night. The focus in this case was on achieving advanced detection and speed reduction in such curves. The second set of improvements was directed toward slowing traffic on rural two-lane undivided roads in small towns during the day. The focus in this case was on achieving traffic calming within the town. The experiment was conducted in the Federal Highway Administration (FHWA) Highway Driving Simulator (HDS). The two sets of potential low-cost safety improvements were combined into a single driving scenario.
Speed reduction in curves yielded the following order of tested treatments (from best to worst): (1) post-mounted delineators (PMDs) enhanced by streaming light-emitting diode (LED) lights slowed drivers down the most by an average of 9 mi/h (14.5 km/h); (2) standard PMDs slowed drivers down by 7 to 8 mi/h (11.3 to 12.9 km/h); and (3) edge lines slowed drivers down by 2 mi/h (3.2 km/h). The same order was obtained for increases in the distance at which drivers were able to identify either the direction or the severity of the curve ahead as follows: streaming LED PMDs increased detection the most (560 to 1,065 ft (171 to 325 m)); standard PMDs increased detection distance by 45 to 200 ft (13.7 to 61 m); and edge lines increased detection distance by zero to 25 ft (zero to 7.6 m). PMDs with edge lines performed better than pavement markings alone. The streaming PMDs solution offered the greatest potential increase in recognition distance.
Speed reduction in towns yielded the following order of tested treatments: (1) chicanes slowed drivers down the most by an average of 4 to 9 mi/h (6.4 to 14.5 km/h); (2) parked cars on both sides of the road slowed drivers down by approximately 4 mi/h (6.4 km/h); and (3) bulb-outs (sometimes known as neck-downs) resulted in only a small speed reduction of 1 mi/h (1.6 km/h) or none at all.
In summary, curves using PMDs with edge lines performed better in terms of slowing drivers down than curves using pavement markings alone. The streaming PMDs solution offered the most dramatic potential benefit in terms of advanced curve detection and is worthy of further research. For towns, chicanes slowed drivers down the most followed by parked cars on both sides of the road. Additional study and consideration should be given to adding painted chicanes to town entrances and providing and encouraging parking in the town. The results of this experiment do not take into account other hazardous factors which exist in the real world since it was performed in a simulated environment. Therefore, field validation is recommended for most of the above findings.
This experiment investigated low-cost visibility enhancements for navigating rural horizontal curves at night. According to the Fatality Analysis Reporting System, of the 37,248 fatal crashes in 2007, 6,495 (17.4 percent) were on horizontal curve sections of two-lane rural roads.(1) Of those, 2,739 (42.2 percent) occurred at night. Previous data indicate that approximately 25 percent of all vehicle miles traveled (VMT) occur at night. Actually, rural local roads may have less than 25 percent of VMT occurring at night. Even if one adjusts for the more conservative total VMT, the exposure rate is more than twice as high at night compared to the day. Thus, fatal crashes on rural curves at night represent an important crash category, and improving the visibility of rural curves at night has been shown to reduce this type of crash.(2)
This experiment also investigated low-cost treatments for reducing speeds on main roads through small towns. The Speed Management Strategic Initiative states that in 2003, 86 percent of speeding-related fatalities occurred on roads that were not interstate highways, and the highest speeding–related fatality rates occurred on local and collector roads where the lowest speed limits were posted.(3) The strategic initiative report also suggests that further research should be conducted to identify and promote engineering measures to better manage speed and to achieve appropriate speeds on main roads through towns not suitable for traditional traffic–calming techniques. The present experiment investigated the speed–calming effects of chicanes located at the beginning and end of a town. Additionally, bulb–outs were evaluated at intersection locations in a town.
Ultimately, the goal for improving safety is to reduce the number of fatalities, injuries, and crashes. This experiment used speed reduction as a safety surrogate measure for crashes since speed is a major contributing factor in run–off–road crashes at horizontal curves as well as in the increased risk of a crash while negotiating small towns due to added hazards such as intersections and pedestrians. In the case of curves, curve detection distance serves as an additional safety surrogate measure for crashes. It is assumed that the further away a driver can detect a curve ahead in the road, the more time the driver has to react to the curve and the less likely the driver is to crash.
This experiment was sponsored by the Low Cost Safety Improvements Pooled Fund Study (PFS) Number TPF–5(099). This PFS was formed to evaluate the safety effectiveness of strategies identified in the National Cooperative Highway Research Program (NCHRP) Report 500 Series Guidance for Implementation of the AASHTO Strategic Highway Safety Plan.(4) This PFS focuses on the evaluation of strategies through rigorous before–after crash evaluations of sites within the United States.(5) In cases where the quality and/or quantity of before–after crash data are/is not sufficient, other methodologies are considered, including the use of safety surrogate data collected from driving simulators.
In June 2007, the Technical Advisory Committee for the PFS met to discuss possible safety improvement strategies that might be evaluated effectively in the FHWA HDS. The selection criteria for study strategies included both the expected relative benefits and costs of each candidate treatment as well as the feasibility of implementing each candidate treatment in the HDS. Two areas were identified for the testing of alternative treatments: (1) visibility improvements to help drivers safely navigate curves in rural roads at night and (2) traffic–calming improvements to slow drivers down when passing through small rural towns during the day. A single laboratory experiment was devised to address both of these research areas, and a draft list of research questions was developed.
The initial set of visibility enhancements that were considered for rural curves included edge lines, retroreflective raised pavement markers (RRPMs), PMDs, and chevrons. However, only a limited number of alternative treatment types could be effectively studied in a single driving simulator experiment. RRPMs were eliminated because the high contrast ratio of newly applied RRPMs was difficult to reproduce in the driving simulator. Chevrons were eliminated because a large body of relatively consistent research literature already exists on that topic. Past research on chevrons has proven their effectiveness for many years. Development of Accident Reduction Factors summarizes many of these earlier studies and data analyses from State databases.(6) From the data, crash reductions ranging anywhere from 20 to 71 percent were found when chevrons were installed.
Edge lines and PMDs were selected as the best alternatives for a single simulator study. Both are relatively easy to simulate and are commonly used to improve driver safety on rural curves as a countermeasure for run–off–road crashes at night. Research shows that introducing and enhancing edge lines has a wide range of results from decreasing vehicle speeds by 3.1 mi/h (5 km/h) to increasing vehicle speeds by 6.6 mi/h (10.6 km/h).(7) Although the overall effect of combining the results of multiple studies was not statistically different from zero, individual experiments under different conditions showed results different from zero. In terms of driver preview distances for curves driven at night, Molino et al. calculated anywhere from a 12– to 70–percent improvement in curve detection distance due to edge lines.(8)
PMDs have also been found to reduce crash rates on relatively sharp curves at night. Highways with PMDs have lower crash rates than those without PMDs.(2) Agent and Creasey performed field and laboratory investigations which indicated that PMDs have a beneficial effect, although pavement markings have an even greater beneficial effect based on vehicle speed and lane encroachment.(9) Meanwhile, Montella evaluated the safety effectiveness of various horizontal curve delineation treatments in Italy.(10) A treatment using sequential flashing beacons was part of this evaluation. When sequential flashing beacons were added to chevrons and curve warning signs, the reported number of crashes decreased by 77 percent compared to the expected number using an Empirical Bayes analysis. A variation of these sequential flashing beacons was investigated in this experiment where simulated reflectorized PMDs enhanced by LED lamps produced a similar sequential streaming pattern of lights.
The current experiment also investigated potential low–cost speed reduction techniques for small towns. The initial set of traffic–calming treatments that may have been applied in small towns included bulb–outs, chicanes, medians, and the presence of parked cars. Medians were eliminated because only a limited number of alternative treatment types could be effectively studied in a single driving simulator experiment. Chicanes and bulb–outs were selected because they are commonly employed to slow drivers down in populated areas. Chicanes are curb extensions that form a series of reverse curves to force a driver to slow down. There are several resources available regarding the design of chicanes, but their effectiveness has not been determined through rigorous research. Traffic Calming: State of the Practice refers to an installation of chicanes and speed tables in Montgomery County, MD, where speeds decreased from 34 to 30 mi/h (54.7 to 48.3 km/h) with volume decreases from 1,500 to 1,390 vehicles per day.(11) In another location where raised crosswalks, raised intersections, and chicanes were applied in Cambridge, MA, speed reductions from 30 to 21 mi/h (48.3 to 33.8 km/h) were observed. It is important to recognize that those analyses consisted of only one location with multiple treatments, so the effects of individual treatments cannot be disaggregated. Traffic Calming: State of the Practice also documents reductions in crash frequencies. However, in cases where traffic volumes also decreased, such data may not be valid. Marek and Walgren found that chicanes were effective at reducing 85th–percentile speeds at four different locations in Seattle, WA, with reductions between 5 and 13 mi/h (8 to 20.9 km/h) inside the chicane area and between 1 and 6 mi/h (1.6 and 9.7 km/h) outside the chicane area (after the chicane had been passed).(12)
Bulb–outs are curb extensions that are generally located at intersections. They are designed to reduce the curb–to–curb roadway width in order to slow drivers down. King analyzed the effect of bulb–outs in New York City, NY, and found that at four of six surveyed locations, the overall severity rates for crashes were reduced after bulb–outs were installed.(13) Furthermore, at two of three locations, the injury severity rate was also reduced. Huang and Cynecki performed an analysis at two locations each in Cambridge , MA , and Seattle , WA , to determine whether drivers would yield to pedestrians with the addition of bulb–out treatments.(14) The results for Cambridge were inconclusive. In Seattle , there was no change in vehicle yielding behavior; however, the researchers did not discuss whether or not drivers reduced speeds in the vicinity of intersections with bulb–outs.
It is possible to combine half bulb–outs and implement them with painted or raised medians. In this manner, the traffic lane can be narrowed by a similar amount, but conflicting traffic can be separated, helping to prevent head–on or sideswipe opposite direction crashes. For chicanes, a similar kind of additional median barrier may be needed to reduce the likelihood of vehicles running into each other. In this instance, truck, off–tracking, and emergency vehicle use become possible issues. However, these combined strategies were beyond the scope of the current experiment.
In summary, the effects of edge lines for reducing speed in curves are inconclusive, but there is some evidence that edge lines can increase curve detection distance. PMDs tend to decrease speed and reduce crashes on curves, and streaming lights have been shown to lower crash rates. This experiment's hypothesis states that both edge lines and PMDs will lower vehicle speeds before and in curves and increase curve detection distances. For speed calming, chicanes have been shown to reduce vehicle speeds, and bulb–outs have been shown to reduce crash rates. However, both of these treatments may pose other safety hazards. The bulb–out compresses the lane width at the intersection, and the chicane deflects and narrows the roadway before the intersection. These configurations may slow down the traffic but may introduce other safety problems. For example, when considering the situation of driving through an intersection and encountering through vehicles from the opposite direction and/or turning vehicles coming from the intersecting street, bulb–outs restrict the traffic lanes and may thereby make the intersection less safe. Similarly, when considering driving through a chicane and encountering through vehicles from the opposite direction, the chicane forces tight turning maneuvers in a constricted area and may thereby make the immediate roadway less safe. Despite these safety concerns, the hypothesis states that both chicanes and bulb–outs will reduce vehicle speeds in small towns. However, when one considers the above safety reservations regarding the implementation of chicanes and bulb–outs, this hypothesis needs to be tested by means of field validation. The simulation environment of this experiment could not effectively represent many inherent dangers of implementing such treatments.
The research questions concerning speed and acceleration for curves are as follows:
The research questions concerning detecting the direction and severity of curves are as follows:
The research questions concerning speed and acceleration for towns are as follows: