Scott Himes

Speed Prediction Models for Multilane Highways: Simultaneous Equations Approach

Past speed-related research has focused on the operational effects of roadway geometrics along rural two-lane highways using ordinary least-squares regression models. More recent research has focused on the association of traffic flow characteristics on vehicle operating speeds along multilane, limited access highways using a simultaneous equations approach. Few research studies, however, have been conducted to determine the combined association between various geometric design features and traffic flow on operating speeds along multilane highways. This research considers both geometric design and traffic flow parameters, in a simultaneous equations framework, to model the mean operating speed and speed deviation on four-lane highways (two lanes in each direction). Models for both left- and right-lane mean speeds and speed deviations were estimated. The three-stage least-squares estimator was used to investigate the possible endogeneity of mean speed and speed deviation in the system of equations and to account for the contemporaneous correlation between the disturbances across the equations. The results indicate that different geometric design features are associated with mean speed and speed deviation in the left- and right-lane models. As such, it is recommended that future multilane highway speed models consider using a simultaneous equations framework.

Exploring the Association between Traffic Safety and Geometric Design Consistency Based on Vehicle Speed Metrics

AbstractPast design consistency research has demonstrated the relationship between operating speeds and geometric design features on two-lane rural highways. However, little is known about the relationship between geometric design consistency and traffic safety. In this study, design consistency is referred to as the difference between operating speed and inferred design speed, and design consistency density is measured to account for the effect of elements upstream and downstream of the study element. To perform the design consistency–safety evaluation in the present study, geometric design, roadway inventory, crash, and operating speed data were collected along two case-study highways in central Pennsylvania (U.S. 322 and PA 350). Several count regression model formulations were used to explore the statistical association between design consistency and total crash frequency. A statistically significant positive association between geometric design consistency and safety was found. Design consistency sur...

Understanding Speed Concepts: Key Definitions and Case Study Examples

The relationships between design speed, operating speed, and posted speed limits are important considerations in the geometric design of highways and streets. Highway designers establish geometric design criteria by using a designated design speed. The ultimate objective is compatibility between design speed, operating speed, and posted speed limit–-or speed harmony. The design speed concept is intended to result in operating speeds that are consistent with the intended function of the highway or street and are therefore favorable with respect to safety and mobility. Because AASHTOs A Policy on Geometric Design of Highway Streets recommends using design values greater than the minimum for a designated design speed and because drivers choose to operate their vehicles according to perceived physical and operational limitations present along a roadway, higher than minimum design values may result in operational inconsistencies–-or speed discord. Speed discord is a condition in which the design speed is lower than the posted speed limit, lower than various operating speed measures, or both. Highway designers may not necessarily perceive such a relationship as problematic, but the public and enforcement personnel may express compliance or safety concerns for highway segments that experience speed discord. This paper provides key definitions of speed concepts used in the geometric design process and presents several case studies that demonstrate the concepts of speed harmony and speed discord.

New Insights on Evaluations of Design Consistency for Two-Lane Highways

Operating speed prediction models along horizontal curves of two-lane rural highways have been the topic of a significant body of literature. Operating speed models for two-lane rural highway tangents and for horizontal curves on low-speed urban streets also exist but are less developed. These models, estimated primarily by means of ordinary least squares regression, generally use aggregate-level data to estimate the effect of geometric design variables on the mean or 85th percentile operating speed but do not explicitly consider measures of speed dispersion (e.g., standard deviation). Most existing models consider speeds of free-flow passenger vehicles; traffic flow effects on speed are generally not quantified. Finally, published literature shows that researchers have not converged on a consistent modeling practice about the use of posted speed limit as an explanatory variable in operating speed models. Collectively, these issues may lead to design consistency assessments that are not truly reflective of the operating conditions along a two-lane highway. These issues are explored in this paper with the use of operating speed data collected at multiple points along eight two-lane highway segments. A simultaneous equations approach is used to estimate a three-equation model of the posted speed limit, mean speed, and speed deviation. The predicted 85th percentile speed is compared with the observed 85th percentile speed along a high-speed, two-lane rural highway. Predicted 85th percentile operating speeds along the same high-speed highway determined with the Interactive Highway Safety Design Model design consistency module are also compared with observed 85th percentile speeds as a basis for evaluating the simultaneous equations approach. Finally, the paper proposes an alternative approach to evaluating design consistency that uses inferred design speed.

Reliability Approach to Horizontal Curve Design

AASHTOs Policy on Geometric Design of Highways and Streets contains geometric design criteria for horizontal curves for new and major reconstruction. These criteria are based on design side friction factors that were established in the 1940s. The objective of this research was to establish a probabilistic approach to the design of horizontal curves and compare the results with current design criteria. Reliability analysis commonly is used in structural engineering and recently has been used in stopping sight distance research for transportation design. In this research, design variables were considered random instead of deterministic to incorporate variations that occurred in the field (e.g., driver heterogeneity or differences in tire performance characteristics) into the analysis. The effects of wet pavements and tire characteristics for passenger cars and heavy trucks were considered. The superelevation rate was considered as a design input, and minimum radii for a target reliability index were presented for combinations of input mean speeds and superelevation rates. All other design inputs were considered random variables (e.g., available pavement friction), along with their appropriate distributions, in the reliability analysis. The results show that a target reliability index of 3.0 is appropriate for use in the probabilistic design of horizontal curves. Furthermore, when only skidding failure is considered, the design criteria for passenger cars can reasonably accommodate heavy trucks. The recommendations for future research consider probabilistic methods and more complex models of horizontal curve forces.

Examining fatal crash reductions by first harmful events since the introduction of the Federal Highway Safety Improvement Program

The federal Highway Safety Improvement Program (HSIP) has been associated with the reduction in fatal crashes since 2006, but the reasons for the reduction remain largely unknown. This paper examines the reduction in fatal crashes in terms of different types of first harmful events that can provide insight into crash causes and prevention strategies. In this study, fatal crashes were categorized into four types: overturn, collision with motor vehicle in transport, collision with fixed object, and collision with nonmotorist. Fixed-effects and mixed-effects Poisson models were used to estimate the magnitudes of fatal crash reduction by first harmful events for each state. Fatal crashes due to collisions with nonmotorists and motor vehicles in transport have been reduced by 10% and 5.3%, respectively, compared with the 2001 to 2005 period. Fatal crashes due to overturn and collision with a fixed object decreased in some states but remained unchanged or increased in other states. Nevertheless, the numbers of national fixed-object and overturn fatal crashes have been reduced by 3% and 0.7%, respectively, as a whole. This study also investigated possibilities that could be associated with the magnitudes of the reductions, for example, the different traffic laws among states. It was found that although different safety improvement projects were implemented to target the various types of crashes, the improvements were also likely to be beneficial to other crash types. These are referred to as spillover effects. Nationally, fatal crashes have decreased since the introduction of the HSIP partly because of the reduction in fatal crashes due to collisions with nonmotorists and motor vehicles in transport and partly because of spillover effects.

Multistate Safety Evaluation of Intersection Conflict Warning Systems

Intersection conflict warning systems (ICWSs) were evaluated under the FHWA Evaluation of Low-Cost Safety Improvements Pooled Fund Study. The ICWS strategy is intended to reduce the crash frequency by alerting drivers of conflicting vehicles on adjacent approaches at unsignalized intersections. The evaluation used a multistate database of geometric, traffic, and crash data for rural four-legged, two-way, stop-controlled intersections equipped with ICWSs in Minnesota, Missouri, and North Carolina. To account for potential selection bias and regression to the mean, an empirical Bayes before–after analysis was conducted by using safety performance functions (SPFs) for reference groups of similar intersections without ICWS installation. These SPFs also controlled for changes in traffic volumes over time and time trends in crash counts unrelated to the strategy. The aggregate results indicate statistically significant crash reductions at the 5% level for all crash types for two-lane-at-two-lane intersections and four-lane-at-two-lane intersections. For two-lane-at-two-lane intersections, the crash modification factors (CMFs) for total crashes, fatal and injury crashes, and right-angle crashes are 0.73, 0.70, and 0.80, respectively, and for four-lane-at-two-lane intersections, they are 0.83, 0.80, and 0.85, respectively. The benefit–cost (B:C) ratio estimated with conservative cost and service life assumptions is 27:1 for all two-lane-at-two-lane intersections and 10:1 for four-lane-at-two-lane intersections with post-mounted warning signs. The results suggest that the strategy, even with conservative assumptions on cost, service life, and the value of a statistical life, can be highly cost-effective. As this strategy is evolving, this study reflects installation practices to date.

Evaluation of Effectiveness of the Federal Highway Safety Improvement Program

The Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users established the Highway Safety Improvement Program (HSIP), which authorized about

Statistical model of in-vehicle sound generated from highway rumble strips

1.3 billion/year from 2006 to 2009 for highway safety projects. The HSIP aims “to achieve a significant reduction in fatalities and serious injuries on all public roads,” and the number of national traffic fatalities seems to have decreased at about the same time. This study sought to evaluate the effectiveness of the HSIP in reducing fatal crashes in the United States. The study adopted fixed-effect panel models and multilevel mixed-effect models to deal with random fluctuations both before and after introduction of the HSIP and state-specific effects. The results show a drop of about 7.5% in national traffic fatalities since introduction of the HSIP compared with the average for 2001 to 2005, but the magnitude of reduction varied by state. States’ safety-related spending did not increase after introduction of the HSIP. Increased federal safety funding was offset by reduced state funding (crowd-out effect). The magnitude of states’ fatal-crash reduction was highly associated with years of available crash data, prioritizing method, and use of roadway inventory data. Moreover, states that prioritized hazardous sites by using more detailed roadway inventory data and the empirical Bayes method had the greatest reductions; all of those states relied heavily on the quality of their crash data systems. This study found that effectiveness of the HSIP in reducing national fatal crashes is very likely attributable to mandated reporting requirements, which helped states allocate safety spending more effectively and efficiently. It also suggests that more consistent and reliable crash data will allow states to employ more sophisticated prioritization methods and make better highway safety investment decisions.

Guidelines for Traffic Control Devices at Changes in Horizontal Alignment

Shoulder and centreline rumble strips are used on highways in the USA to prevent single-vehicle run-off-road and opposite direction crashes. Both rumble strip types have been shown to provide positive safety benefits on a variety of roadway types. The elevated in-vehicle sound and vibration levels produced by rumble strip patterns provide the alerting properties to warn drivers that their vehicles have left the intended travel lane. This study estimated a model of in-vehicle sound intensity, frequency, and duration using seemingly unrelated regression. The statistical model indicates that increasing the vehicle speed; rumble strip length, width, and groove depth; and using a milled versus a rolled rumble strip pattern, all increase the in-vehicle sound level relative to the ambient level. A rumble strip on the right-side of the travel lane; increasing the vehicle angle of departure; increasing the centre-to-centre spacing of the grooves; a concrete roadway surface; and a wet roadway surface, all decrease the in-vehicle sound relative to the ambient sound.