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56 C h a p t e r 1 0 Summary Over half of motor-vehicle fatalities are roadway depar- tures. Rural horizontal curves are of particular interest because they have been correlated with overall increased crash occurrence. Although transportation agencies expend significant resources to address the problem, the interaction between the driver and roadway environment is not well understood. As a result, it is difficult to select appropriate countermeasures. To address this knowledge gap, data from the SHRP 2 NDS and RID were used to develop relationships between driver, roadway, and environmental characteristics and risk of a road departure on rural curves. Only curves on rural two-lane paved roadways with posted speed limits of 64 km/h to 97 km/h (40 mph to 60 mph) were included. The research was tailored to address four fundamental research questions: 1. What defines the curve area of influence? 2. What defines normal behavior on curves? 3. What is the relationship between driver distractions; other driver, roadway, and environmental characteristics; and risk of roadway departure? 4. Can lane position at a particular state be predicted as a function of position in a prior state? Each question addresses the problem from a different per- spective; as a result, a different methodology was proposed for each, as described in the corresponding sections. The team identified rural curves of interest using the RID and requested time series data from the DAS, which pro- vided vehicle kinematics (e.g., speed, acceleration) for those curves. A forward roadway view was also provided. Vehicle, roadway, and environmental data were extracted and used as variables in the various analyses. Eyes-off-roadway distrac- tions and driver glance locations were reduced using the driver face and steering wheel video data at the VTTI secure data enclave. Crash surrogates were used because crashes/near crashes had not been coded at the time this research was conducted. A number of potential crash surrogates were considered against the data available and the expected accuracy of rele- vant variables in the NDS data (e.g., lane position, forward radar, vehicle position). Lane offset was the best crash surro- gate, but lane offset was not reliable in a number of traces. As a result, it was used for Research Questions 2 and 4, resulting in a smaller sample of data for those research questions. Because offset was not reliable in a number of traces, it was determined that encroachments would be the best crash sur- rogate for Research Question 3. A right-side encroachment is defined as the right side of the vehicle crossing the right lane line, and a left-side encroachment is defined as the left side of the vehicle crossing the centerline. Discussion and recommendations for Countermeasures Data from the SHRP 2 NDS and RID were used to develop relationships between driver, roadway, and environmental characteristics and risk of a roadway departure on rural two- lane curves on paved roadways. The four research questions addressed the problem from different perspectives, and a different methodology was devel- oped specific to each. The analytical method, data sampling and segmentation approach, general variables considered, results, and implications are discussed in the corresponding sections. In general, the research questions covered three areas: curve area of influence, lane position, and speed. Summary and Recommendations
57 Curve Area of Influence Identifying the curve area of interest was addressed in Research Question 1. Regression and Bayesian analyses were used to model the point (upstream of the PC) at which the driver reacts to the curve. Reaction point was inferred as the point at which speed or gas pedal position changed significantly from upstream driving. Results showed that, depending on radius of curve, drivers begin reacting to the curve 164 m to 180 m (538.1 ft to 590.6 ft) upstream of the point of curvature. Reaction point was com- pared with sign placement guidelines in the Manual on Uni- form Traffic Control Devices (Federal Highway Administration 2009). It was determined these guidelines are appropriately set based on where drivers actually react to the curve. Research Question 1 also found that drivers begin reacting to the curve sooner for curves with larger radii than for curves with smaller radii. Drivers may not be able to gauge the sharpness of the curve, or sight distance issues may be a con- cern for sharper curves. This suggests that use of counter- measures, such as chevrons or RPMs, that better delineate the curve may provide better advance information for drivers. It should be noted that the model only identified where drivers reacted to the curve. This research question did not attempt to answer whether the reaction point was sufficient for drivers to successfully negotiate the curve. Lane Position Lane position was modeled in Research Questions 2, 3, and 4. Offset from lane position was modeled for normal driving in Research Question 2, and probability of a right-side or left-side encroachment was modeled in Research Question 3. Research Question 4 used time series data to model driver behavior at 0.1-s intervals based on driver, traffic, and roadway character- istics. However, the objective of Research Question 4 was to demonstrate the utility of the approach, and only limited data were used in the analyses. Several driver factors are related to lane position. Research Question 2 found that offset from the center of the lane within the curve is influenced by offset upstream of the curve. Results from Research Questions 2 and 3 indicate that offset and likelihood of an encroachment are correlated to glances away from the forward roadway and glances associated with a dis- traction. Males are more likely to have a left-side encroach- ment, and younger drivers are more likely to deviate within their lane. Research Question 2 also indicated that offset from the center of the lane varies with position with the curve. Research Questions 2 and 3 found that behavior differs when driving on the inside versus the outside of the curve (from the perspective of the driver). Because there are natural variations in posi- tion along the curve, drivers may be more vulnerable to lane departures at certain points in the curve. These results sug- gest that countermeasures such as rumble strips, paved shoulders, and high-friction treatments may ameliorate the consequences of variations in lane position through the curve. Several roadway characteristics were correlated to lane posi- tion. Research Question 2 found that nighttime driving was a factor in driver lane position, with offset tending more toward the left of the lane center. Results from Research Question 3 showed a correlation between radius and the probability of encroachment, but the effect was small. These results also indi- cated that drivers were more likely to have a right-side encroach- ment on curves when an advisory sign was present but less likely when a guardrail was present along the curve. It should be noted that advisory signs and guardrails are used on certain types of curves, and, as such, either may be a surrogate for a certain type of curve. These results suggest that presence of advisory signs do not in and of themselves mitigate roadway departures. Addition- ally, drivers may adjust their speed when the roadway sug- gests a more dangerous situation (i.e., presence of a guardrail suggests an unforgiving roadside environment). Consequently, better curve delineation may allow drivers to better gauge upcoming changes in roadway geometry, resulting in better speed selection and decreased risk of a roadway departure, and may help decrease speed. Delineation countermeasures include chevrons, the addition of reflective panels to exist- ing chevron posts, reflective barrier delineation, RPMs, post- mounted delineators, edge lines, and wider edge lines. Speed Driver speed near the curve entry was modeled in Research Question 2, and probability of a driver exceeding the advisory speed (if present) or posted speed (if not present) by 8 km/h and 16 km/h (5 mph and 10 mph) was modeled in Research Question 3. Several driver characteristics were related to speed. Both Research Questions 2 and 3 indicated that drivers traveling at higher speeds in the tangent section were also likely to speed within the curve, and younger drivers and pick-up truck/SUV drivers were more likely to speed. Both research questions also found that drivers looking away from the roadway had slower speeds within the curve. Several roadway characteristics were correlated to speed. Results from Research Question 3 showed that drivers are slightly more likely to exceed the curve advisory/posted speed limit as radius of curve decreases, but this may be because sharper curves have lower advisory speeds. Results also showed
58 that drivers are less likely to speed when paved shoulders or RPMs are present. The probability that a driver would exceed the posted/advisory speed by 8 km/h (5 mph) or more was higher for curves with obscured/missing edge lines. When roadway characteristics are considered together, the results may suggest that appropriate delineation as pro- vided by RPMs and presence of edge lines may provide cues to drivers, allowing them to better gauge the sharpness of the curve and select appropriate speeds. Delineation counter- measures include chevrons, the addition of reflective panels to existing chevron posts, reflective barrier delineation, RPMs, post-mounted delineators, edge lines, and wider edge lines. The speed models also suggest that driver age and upstream speed have a significant impact on driversâ speed within a curve. As a result, speed management countermeasures that affect tangent speed will also decrease curve speeds. The model also indicates that speed management is appropriate to get driversâ attention before entering a curve. Countermeasures specifically targeted to reduce speed on curves include dynamic speed feedback signs, on-pavement curve warning signs, and flashing beacons.
