Transportation Research Record: Journal of the Transportation Research Board

The association between horizontal curve design (e.g., radius and type) on rural, two-lane, undivided highways and motorcycle crash frequency is not well documented in existing reports and publications. This study aimed to investigate the effects of design parameters and associated factors on the occurrence of motorcycle crashes with consideration of the issue of unobserved heterogeneity. A random-parameters negative binomial regression model was developed on the basis of data on 431 motorcycle crashes, which were collected on 2,179 horizontal curves along two-lane, undivided highways in Florida for 11 years (2005 to 2015). Four normally distributed random parameters (i.e., logarithm of curve radius, reverse curves, pavement condition, and rough pavement indicator) were identified to represent their heterogeneity caused by unobserved factors over time, space, individuals, or some combination thereof. The major conclusions are the following: (a) an increase in curve radius, on average, significantly and near-logarithmically reduced motorcycle crash frequency on rural, two-lane, undivided highways (this effect was more significant when the curve radius was less than 2,000 ft); (b) 74.8% of reverse curves tended to reduce motorcycle crash frequency on rural, two-lane, undivided highways (for the remaining 25.2%, the effect had an opposite effect; on average, the likelihood of motorcycle crashes on reverse curves decreased by 39%); (c) the crash modification function (CMF) for curve radius on rural, two-lane, undivided highways was established, given the radius of 5,000 ft as the baseline, as a power formula, CMF = (radius/5,000)^-0.208.

Data on traffic volumes are required to estimate the safety performance functions (SPFs) used to develop crash modification factors and for various safety management applications. Estimation of SPFs for motorcycle crashes can be especially challenging because few jurisdictions collect motorcycle traffic volume data systematically. To address this challenge, analyses with data from Florida, Pennsylvania, and Virginia were conducted to explore how much predictive power for an SPF was lost when motorcycle traffic volumes were not known. The results of the analyses showed that when motorcycle volumes were unknown, the use of total annual average daily traffic on its own was sufficient to develop motorcycle crash SPFs. The potential bias from missing motorcycle-specific annual average daily traffic was sufficiently negligible where it existed, not to preclude SPF development. A more significant issue in the development of motorcycle crash SPFs is to work with a crash type that is relatively rare, so that SPFs cannot be developed for all motorcycle crash types or site types.

Roundabouts reduce fatal and injury crashes at intersections when converted from other intersection control types. In Wisconsin, roundabouts have been linked to a 38% decrease in fatal and injury crashes. Part of this reduction can be attributed to crash types that result in the mitigation of more serious injuries. However, the reduction comes at a cost because other crash types, such as single-vehicle collisions, may increase. Six years of crash data on 53 roundabouts in Wisconsin were examined for crash causes and geometric characteristics that affected single-vehicle crashes. Weather and impaired driving, particularly by younger drivers, were primary causes for more than half of all single-vehicle crashes at the study roundabouts. Younger drivers (18 to 24 years of age) were involved in a significantly higher proportion of single-vehicle crashes than the total proportion of licensed drivers in that age group. Younger drivers were involved in approximately one-third of all crashes that involved impaired driving and in two-thirds of all speed-related single-vehicle crashes. A negative binomial model was constructed to estimate run-off-road crashes at approaches. It was found that roundabouts with higher approach speeds and higher traffic volumes experienced more run-off-road crashes. Landscaped central islands experienced significantly lower frequencies of run-off-road crashes.

A motorcyclist helmet is considered important safety equipment because it prevents or minimizes head and brain injuries, which are often fatal. Hence, in the 1960s and 1970s, most of the states in the United States enacted the universal helmet law (UHL) requiring all motorcyclists to wear helmets. Many researchers have examined the effect of the helmet law changes by using before-and-after studies and found that repealing the law had a negative effect on motorcyclists. In this study, the authors have attempted to explore the long-term impacts of repeal and reinstatement of the UHL by using 13 to 16 years of data. A before-and-after study with a comparison group and empirical Bayes methods was adopted to account for the passage of time and its effect on other factors such as exposure, maturation, trend, and regression-to-the-mean bias. A range of safety performance functions was developed on the basis of counties and parishes, and the expected fatal motorcycle crashes were calculated. The results showed that the UHL repeal still had significant effects on motorcycle fatal crash counts even 7 to 12 years after the repeal of the law. The crash modification factors showed that the UHL repeal increased the number of motorcycle fatal crashes by 15% to 41%, whereas reinstatement of the UHL decreased it by 21% to 27%. It is expected that the results from this study could be helpful for state policy makers to clearly understand the effects of the UHL on reducing motorcycle fatal crashes.

Horizontal curves have been of great interest to transportation researchers because of expected safety hazards for motorcyclists. The impacts of horizontal curve design on motorcycle crash injuries are not well documented in previous studies. The current study aimed to investigate and to quantify the effects of horizontal curve design and associated factors on the injury severity of single-motorcycle crashes with consideration of the issue of unobserved heterogeneity. A mixed-effects logistic model was developed on the basis of 2,168 single-motorcycle crashes, which were collected on 8,597 horizontal curves in Florida for a period of 11 years (2005 to 2015). Four normally distributed random parameters (moderate curves, reverse curves, older riders, and male riders) were identified. The modeling results showed that sharp curves (radius <1,500 ft) compared with flat curves (radius ≥4,000 ft) tended to increase significantly the probability of severe injury (fatal or incapacitating injury) by 7.7%. In total, 63.8% of single-motorcycle crashes occurring on reverse curves are more likely to result in severe injury, and the remaining 26.2% are less likely to result in severe injury. Motorcyclist safety compensation behaviors (psychologically feeling safe, and then riding aggressively, or vice versa) may result in counterintuitive effects (e.g., vegetation and paved medians, full-access-controlled roads, and pavement conditions) or random parameters (e.g., moderate curve and reverse curve). Other significant factors include lighting conditions (darkness and darkness with lights), weekends, speed or speeding, collision type, alcohol or drug impairment, rider age, and helmet use.

Many rigorous studies have shown the effectiveness of helmet use in reducing severe injuries and fatalities to motorcyclists. NHTSA reports estimates of lives saved by U.S. motorcyclists wearing or potentially wearing helmets. This calculation uses a statistically derived estimate of helmet effectiveness in preventing deaths to motorcyclists involved in crashes. Two parts of the estimate of lives saved are (a) deaths prevented assuming crashes occurred that did not kill helmeted motorcyclists in proportion to those that did kill helmeted motorcyclists and (b) deaths that may have been prevented if unhelmeted motorcyclists killed in crashes had been helmeted. Similar estimates can be made for injuries. More readily available crash data enable analysts to investigate whether crashes in their jurisdictions support the inherent assumptions of these estimates. Moreover, the estimates can be stratified to include other crash characteristics such as crash type and rider gender and age. This study compared estimates of potential fatality and injury reductions based on NHTSA averages with estimates with the use of all reported crashes on Colorado state and federal roads in the years 2006 to 2014. Helmet effectiveness is found to depend on crash characteristics in Colorado and does vary from national averages.

Studies investigating crash rates by roadway classification are few and far between and even more rare if extended to focus on heavy vehicles. This study explored and compared two advanced econometric methods—random-parameter Tobit regression and latent class Tobit regression—to determine contributing factors for heavy-vehicle crashes per million vehicle miles traveled while accounting for the unobserved heterogeneity present in crash data. The increasing crash rates in Idaho, crash proportion by roadway classification, and available data made an ideal case study. Empirical results show that although the random-parameter Tobit regression model provides better insight into heavy-vehicle crash rates than the fixed-parameter approach, the latent class Tobit regression model is the preferred methodology for the given data set. Traffic volumes, roadway characteristics, and traffic control devices were among the variables found to be statistically significant. Results from this study provide an alternate framework to account for heterogeneity while identifying key factors by roadway classification that influence heavy-vehicle crash rates. The illustrated framework and analysis by roadway classification can provide guidance to transportation agencies and policy makers and prompt future studies to include a latent class analysis, analysis by road classification, or both.

A series of studies was done at a large truck driver-training center as part of its ongoing cycle of development and implementation of simulator-based training. This project was motivated by the need to accelerate the learning of basic vehicle control skills to allow more time for safety-critical skills. The first studies focused on gearshifting and the second on backing maneuvers. All studies measured the transfer of driving skills learned in the truck simulator to the real truck. The results show that basic vehicle control skills learned on the truck simulator transferred to a real truck and that the learning time required on the truck simulator compared with the real truck was at least equal if not shorter in many cases. A major strength of this project is to complete integration into the normal operations of the training school’s program by using regular teachers and students. The only nonroutine aspects were the extra performance measurements by independent evaluators. Naturalistic studies are prone to confounding factors. Therefore, the consistent evidence of transfer of basic skills from the truck simulator to the real truck across multiple phases with slightly different designs demonstrates the robustness of the findings. Given the success of simulator-based training in aviation, the evidence of successful transfer of training in this study is not surprising. The greater challenge is the future integration of efficient truck-driving simulator-based learning into existing training programs to allow additional time to learn more safety-critical driving skills.

A bus in motion is vulnerable to strong aerodynamic forces that may interfere with its controllability and operational safety. On mountainous highways—especially highways along deep gorges—or under extreme weather conditions, the lateral wind distribution is not always uniform and consequently produces the Bernoulli effect. The objective of this study was to investigate its specific impact on bus safety by using a hybrid method of computational fluid dynamics (CFD) and kinematics simulations. An ordinary bus was chosen as the analysis object, and its three-dimensional model was established in SolidWorks. CFD experiments were conducted in ANSYS/Fluent to estimate the lateral aerodynamic forces. The smart driving system based on the directional control model was built in MATLAB/Simulink to replicate the average driving experience. Then a cosimulation of Adams/Car with Simulink was implemented to quantify and assess the bus handling and stability performance. The CFD simulations report that the aerodynamic force induced by the Bernoulli effect is not negligible for an ordinary bus. As the gradient of wind speed variation increases to over 5 m/s per meter in the lateral direction, the ratio of the aerodynamic force to its gravity exceeds 2%. Kinematics simulations prove that the bus lateral chassis acceleration and the lateral displacement both reached alarm values under the adverse conditions. The study is expected to offer a useful tool and reference for bus accident prevention and analysis in strong wind zones with frequent asymmetric winds. It may also help improve vehicle handling and stability design, especially for oversized vehicles.

As of November 2016, Montana was the only state to maintain a differential speed limit on two-lane two-way rural highways, with a daytime statutory speed limit of 70 mph for cars and light trucks and 60 mph for trucks exceeding a 1-ton payload capacity. Although differential speed limits are common on freeways, the use of differential limits on two-lane roadways presents unique safety and operational issues because of passing limitations and the resultant platooning that occurs. Given these concerns, the speed limit was changed from the differential 70 mph to 60 mph speed limit to a uniform 65 mph limit for all vehicles along 55 mi of two-lane highway in April 2013. This study evaluated the short-term operational and safety impacts associated with this transition. Given the limited time period over which data were available following the speed limit change, surrogate safety measures were investigated to provide immediate feedback as to these impacts. A series of field studies was performed on two-lane rural highways in Montana, which predominantly possessed the 70 mph to 60 mph differential speed limit, as well as on selected locations along the 55 mi where the uniform speed limit was implemented. The locations with 65 mph speed limits generally exhibited shorter platoon lengths and less high-risk passing behavior. Overall, the preliminary findings provide some general support for transitioning to a uniform 65 mph speed limit on two-lane rural highways.

Roundabouts can be a solution to safety concerns common to conventional intersections. Recently in the United States, there has been an increase in the conversion of problematic intersections to roundabouts to improve their safety. However, there are some important considerations and challenges to make roundabouts safe for all users, especially multilane roundabouts. There may also be challenges for drivers to navigate a newly built or a new type of roundabout. Roundabout features such as pavement markings and signage play an important role in driver navigational performance. This research was an effort to evaluate new roundabout design and existing roundabout safety and operational features such as signs and pavement markings. The focus was on how such features influenced the performance of drivers, specifically at multilane roundabouts. Two-lane roundabouts and rotor turbo roundabouts were evaluated. A driving simulator was employed to test various simulation scenarios in a virtual world. The results indicate that lane keeping and higher navigation speeds are still problems at multilane roundabouts. The rotor turbo roundabout outperforms the two-lane roundabout in enabling correct lane choice and navigation speed control by drivers. Furthermore, it was found that roundabout signs and pavement markings used in the United States can be adopted for turbo roundabouts.

In the United States, roundabouts have recently been constructed to replace signalized intersections at freeway ramp terminals as both a safety and an operational treatment. In practice, this treatment is in need of guidelines specifying conditions when the roundabout or signalized intersection is more appropriate to assist practitioners in deciding which alternative to choose. In particular, research providing a comprehensive operational comparison between roundabouts and signalized interchanges is lacking. The current research—though a strictly calibrated microscopic simulation platform—analyzes and models the control delay at double-lane roundabouts and signalized interchanges. Both roundabouts and signalized interchanges were modeled in a Vissim simulation platform. Capacity at each roundabout entrance was calibrated and validated separately for passenger cars and heavy vehicles, since both vehicle types have different critical and follow-up headways. The design of the simulation experiments covered 2,880 different scenarios for roundabouts and signalized interchanges with varying ramp and arterial volumes, ramp spacing, and heavy-vehicle percentages. From the simulation results, control delay and level of service of the off-ramp and arterial approaches of roundabouts and signalized diamond interchanges were modeled and compared. Ultimately, guidelines for the selection between double-lane roundabouts and signalized interchanges were developed and presented in the form of look-up tables. These tables provide an easy-to-use tool for practitioners to determine the appropriate double-lane interchange to install under specific combinations of traffic demand, heavy-vehicle percentage, and ramp spacing conditions.