John McFadden

Evaluating horizontal alignment design consistency of two-lane rural highways : Development of new procedure

Design consistency refers to the condition wherein the roadway geometry does not violate driver expectations. Operating-speed profile models are used to evaluate the consistency of a design by identifying locations with large speed variability between successive design elements. There is a direct correlation between safety and variability in speeds. Recent operating-speed models predict the 85th percentile speeds on horizontal curves and compare this value with the expected 85th percentile speed on the approach tangent. There is a direct correlation between speed variability between successive design elements and crash rates. Eighty-fifth percentile speeds, however, do not necessarily represent the speed reductions experienced by drivers. The primary objective of the research was to assess the efficacy of the use of 85th percentile speed by operating-speed profile models to evaluate the consistency of a design. Speed data were collected at 21 horizontal curve sites. These data were used to evaluate the implication of using 85th percentile speed for evaluating design consistency. A new parameter was investigated for analyzing design consistency: the 85th percentile maximum reduction in speed (85MSR). This parameter is calculated by using each driver’s speed profile from an approach tangent through a horizontal curve and determining the maximum speed reduction each driver experiences. These maximum speed reductions are sorted, and the 85th percentile value becomes the statistic of interest, or 85MSR. 85MSR was compared with the difference in 85th percentile speeds (85S), and it was found that 85MSR is significantly larger than 85S. The data showed that, on average, 85MSR is approximately two times larger than 85S. Models were developed that predict 85MSR as a function of geometric design elements, and these models could be used to complement existing operating-speed models.

Application of Artificial Neural Networks to Predict Speeds on Two-Lane Rural Highways

The ability to predict accurately vehicular operating speeds is useful for evaluating the planning, design, traffic operations, and safety of roadways. Operating speed profile (OSP) models are used in the geometric design of highways to evaluate design consistency. Design consistency refers to the condition where the geometric alignment does not violate driver expectations. Existing OSP models have been developed using ordinary linear regression methods. However, the assumptions and limitations inherent to linear regression may at the very least complicate model formulation. If not acknowledged and corrected for, deviations from these assumptions can also adversely affect the efficacies of such models. Artificial neural networks (ANNs) are modeling tools that do not impose the stringent assumptions and limitations imposed by regression. It is therefore of interest to know whether ANNs are viable alter natives to linear regression for OSP modeling. Two backpropagation ANNs for operating speed predictions for passenger cars on two-lane rural highways are evaluated, and their performances with regression-based models are compared. The results of these comparisons indicate that the explanatory powers of the ANN models are comparable with those developed by regression. The predictive powers of the two types of models were observed to be comparable, and ANNs were not limited by distributional or other constraints inherent to regression. Therefore, ANNs were determined to be a viable alternative to regression for OSP model construction.

SAFETY, DELAY, AND CAPACITY OF SINGLE-LANE ROUNDABOUTS IN THE UNITED STATES

Roundabouts are a form of at-grade intersection control that is used frequently around the world and is becoming popular in the United States. Roundabouts are being used to replace two-way and all-way stopcontrolled intersections and traffic signals in the United States. Roundabouts have also been used recently in Vail, Colorado, to improve an existing stop-controlled freeway interchange system. In this study, five single-lane roundabouts are studied to assess their safety and operational performance. All five sites were stop-controlled before roundabouts were installed, and overall the sites experienced a reduction in accident frequencies, rates, and also control delay. Aside from a review of safety and delay data before and after installation of the roundabouts, this study includes a comparison of field-measured control delay with that predicted by SIDRA, an analytically based software package that can analyze at-grade sign- and signal-controlled intersections as well as roundabouts. This study will help agencies better understand their ability to predict delay at American roundabouts. Finally, findings are presented regarding the accuracy of the roundabout capacity model contained in the 1997 update to Chapter 10 of the Highway Capacity Manual. Because of the lack of roundabout entries that are operating at capacity in the United States, an approximation of potential capacity based on available gaps in the circulating stream was made. These findings indicate that the manual may be optimistic in its prediction of capacity for single-lane roundabouts in the United States; however, it should be noted that the lack of roundabout entries operating under capacity in the United States only allows for an approximation of field capacity to be made at this time.

Automated Enforcement of Red Light Running Technology and Programs: A Review

Automated enforcement involves the use of image capture technology to monitor and enforce traffic control laws, regulations, or restrictions. The increase in aggressive driving and the high percentage of crashes that occur at intersections led to the development and implementation of automated enforcement technology to detect and cite motorists who enter a signalized intersection in violation of the red phase. The primary focus of this research was to establish how well the automated enforcement system achieves its principal objective: reducing crashes and red light running (RLR) violations at signalized intersections. Evaluations of automated enforcement programs at three locations in the United States were performed as part of this research. The automated enforcement programs in New York City; Polk County, Florida; and Howard County, Maryland, were reviewed as a part of this research. Some of the major findings from this research are as follows: (a) In 1997 there were over 789,000 crashes at signalized intersections, of which 97,000 were attributed to RLR; (b) In 1997, 961 deaths were attributed to RLR; (c) Electronic enforcement is a proven technique used globally to curb RLR violations and crashes; (d) A synthesis of automated RLR enforcement programs in the United States showed promising results; (e) New York City has the oldest automated RLR enforcement program in the United States and has yielded a 20 percent reduction in violations since 1993; (f) Although additional data quantifying the effect of automated RLR enforcement campaigns are needed for Polk and Howard counties, preliminary findings are promising; (g) A 10-step process for successful implementation of an automated RLR enforcement program was illustrated.

Formulation and Validation of Operating Speed-Based Design Consistency Models by Bootstrapping

Current U.S. policy for designing rural two-lane highways is based on design speed to ensure consistency among consecutive highway segments. The design speed concept, however, does not ensure that a consistent alignment will be achieved. A recent FHWA-sponsored project (Horizontal Alignment Design Consistency for Rural Two-Lane Highways) led to three operating speed-based geometric design consistency models, which have not yet been validated. Traditionally, the validation of such models involves the collection of additional data. The statistical technique known as “bootstrapping” was used to formulate and validate the operating speed-based geometric design consistency models by using the existing FHWA database. Bootstrapping involves random sampling with replacement from the existing database, which becomes the population. One-half of the original data collected are used in formulating the models. The remaining half of the data are subsequently used for validation. The models resulting from bootstrapping were statistically equivalent to the models developed in the FHWA study. In addition, the model validation indicated that the bootstrapping technique used to validate the operating speed models is a viable alternative means of validation. It was concluded that bootstrapping is a very useful tool that can be exploited in many related areas in the transportation field, especially because of the large amounts of data typically required in developing and validating empirical models.

Analysis of Sight Distance, Crash Rate, and Operating Speed Relationships for Low-Volume Single-Lane Roundabouts in the United States

AbstractThis paper explores the relationship between sight distance parameters, crash rates, and operating speeds at low-volume single-lane roundabouts in the United States. The understanding of the interaction of design, operations, and crash performance is a step forward in the development and application of performance-based standards for roundabouts. The specific objective of this paper is to quantify the relationship between crash rates, sight distance parameters, and operating speeds to present an approach to establishing performance-based standards that highway practitioners can adopt in roundabout design. Geometric, traffic, and crash data were collected on 72 approaches to 19 low-volume single-lane roundabouts in six states. The data for these sites were broken into two groups based on the posted speed limit (at 40  km/h and greater than 40  km/h). In addition, the associations between different sight distance parameters, crash parameters, and operating speed data were investigated. The research ...

Evaluation of Inexpensive Global Positioning System Units to Improve Crash Location Data

The state of Alabama stores and manages detailed information about vehicle crashes including their approximate locations. Currently, crash locations are estimated and hand recorded by highway patrol officers at the scene of a crash. These locations are typically placed at identifiable points along the roadway, such as mileposts and intersections. The resulting resolution can place vehicle crashes caused by the same roadway variable as far apart as 2 km, making accurate identification of roadway safety hazards difficult. The research project was initiated through the University Transportation Center for Alabama to improve vehicle crash analysis through accurate and inexpensive collection and display of crash locations. The research employed Global Positioning System (GPS) and Geographic Information System (GIS) technologies. GPS units ranging in price from

Use of Monte Carlo Simulation for a Sensitivity Analysis of Highway Safety Manual Calibration Factors

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Distribution of roadway geometric design features critical to accommodation of large trucks

3,400 were evaluated on the basis of cost, accuracy, usability, additional equipment needed, and time required to collect a location. From this analysis two GPS units were selected: (a) a handheld unit, the Garmin GPS 12 Map, and (b) a computer-based unit, the Rand McNally StreetFinder Deluxe. With the recent removal of selective availability, the research has shown that inexpensive GPS units tested are capable of locating a vehicle crash within an 8-m radius. In addition, both an off-the-shelf GIS and accurate basemaps were employed to display the new GPS crash data. By analyzing GPS locations in a GIS, “hot spots” where multiple crashes occurred were identified through buffering operations. An analysis of actual crash location data showed that GPS data provided more information about a crash than conventional methods, specifically at interchanges and along roadway segments.

Influence of Sight Distance on Crash Rates and Operating Speeds at Low-Volume Single-Lane Roundabouts in the United States

The Highway Safety Manual (HSM) is a comprehensive traffic safety tool that allows administrators, planners, engineers, and other agency representatives to make informed decisions about programs for highway safety improvement. Part C of the HSM contains predictive models for quantifying the safety of roadways and intersections; these models require significant data inputs. Because the models were developed with data from specific states, the HSM recommends the use of a calibration factor to account for local conditions when a model is applied elsewhere. The calibration process uses a set of sites and criteria for calculating the calibration factor. The accuracy and the precision of these models depend on the calibration process that develops the factor. This study evaluated the HSM calibration criterias guidance for two-lane, two-way undivided urban arterial roadways and examined a method for improving the accuracy and precision of calibration factors. A sensitivity analysis used calibration sets of various sizes in a Monte Carlo simulation to resample sites. Results indicated that the current HSM criterion of 30 to 50 sites was insufficient for evaluating locations with many sites and that the minimum threshold of observed crashes per year should be reconsidered. A traffic volume analysis revealed a strong correlation between the traffic volume distribution of all sites in a jurisdiction in comparison with selected sites. This relationship was directly linked to the derivation of high-quality calibration factors. The ratio formula for the calibration factor in Part C, Appendix A, of the HSM was validated through another site allocation strategy. The findings provide prompts for future research into enhancing the HSM calibration process.