The state of the practice in highway geometric design consistency was determined through a review of US and foreign geometric design policy, practice and research. Models and a menu-driven microcomputer procedure for their use, were developed for operating-speed and driver-workload consistency evaluations of rural two-lane highway horizontal alignments. The operating-speed model was calibrated based upon speed and geometry data for 138 horizontal curves and 78 of their approach tangents in 5 states. The driver workload model was calibrated based upon two occluded vision test studies on a total of 55 subjects. The operating-speed data suggest that 85th percentile speeds generally exceed the design speed of horizontal curves whose design speed is less than drivers' desired speed (ie 85th percentile speed on long tangents). A preliminary evaluation comparing model-estimated operating-speed reductions versus degree of curvature as predictors of accident experience was conducted using a data base of 1,126 curve sites in three states. The evaluation suggests that accident experience increases as the required speed reduction from an approach tangent to a horizontal curve increases.

https://rosap.ntl.bts.gov/view/dot/740/dot_740_DS1.pdf

Horizontal alignment design consistency for rural two-lane highways

Automated vehicles have received much attention recently, particularly the DARPA Urban Challenge vehicles, Google's self-driving cars, and various others from auto manufacturers. These vehicles have the potential to significantly reduce crashes and improve roadway efficiency by automating the responsibilities of the driver. Still, automated vehicles are expected to crash occasionally, even when all sensors, vehicle control components, and algorithms function perfectly. If a human driver is unable to take control in time, a computer will be responsible for pre-crash behavior. Unlike other automated vehicles--such as aircraft, where every collision is catastrophic, and guided track systems, which can only avoid collisions in one dimension--automated roadway vehicles can predict various crash trajectory alternatives and select a path with the lowest damage or likelihood of collision. In some situations, the preferred path may be ambiguous. This study investigates automated vehicle crashing and concludes the following: (1) automated vehicles will almost certainly crash, (2) an automated vehicle's decisions preceding certain crashes will have a moral component, and (3) there is no obvious way to effectively encode complex human morals in software. A three-phase approach to developing ethical crashing algorithms is presented, consisting of a rational approach, an artificial intelligence approach, and a natural language requirement. The phases are theoretical and should be implemented as the technology becomes available.

Ethical Decision Making During Automated Vehicle Crashes

In some states, frontage roads are frequently used along freeway and fully controlled principal arterial corridors. Their primary function is to provide access between the arterial, or freeway, and adjacent developed property. Little information about the safety performance of rural frontage roads exists. This paper describes the development of a safety performance function (SPF) and accident modification factors (AMFs) for rural frontage road segments. Both one-way and two-way frontage road operations are addressed. The AMFs quantify the relationship between specific changes in geometric design and road safety. The findings from this research show that wider lanes and shoulders are associated with a reduction in segment-related collisions. In addition, the data suggest that the presence of edge marking has an impact on rural two-way frontage road safety. The SPF developed for this research indicates that, for the same traffic volume, rural frontage road segments experience about the same number of severe crashes as typical rural two-lane highways.

https://ceprofs.civil.tamu.edu/dlord/Papers/Lord_and_Bonneson_TRB_2007.pdf

Development of Accident Modification Factors for Rural Frontage Road Segments in Texas

Prediction and estimation of speeds on two-lane rural highways are of enormous significance to planners and designers. Estimation of speeds on curves may be easier than prediction of speeds on tangent sections because of the strong correlation of speeds with a few defined and limiting variables, such as curvature, superelevation, and the side-friction coefficients between road surface and tires. On tangent sections, however, the speed of vehicles is dependent on a wide array of roadway characteristics, such as the length of the tangent section, the radius of the curve before and after the section, cross-section elements, vertical alignment, general terrain, and available sight distance. Few studies have dealt with this issue because a considerable database is necessary to identify any significant trends and substantial modeling effort is required. Research analyzed the variability of the operating speeds on 162 tangent sections of two-lane rural highways, and models were developed for prediction of operat...

Predicting Operating Speeds on Tangent Sections of Two-Lane Rural Highways:

Passenger car equivalents (PCEs) have been used extensively in the Highway Capacity Manual to establish the impact of trucks, buses, and recreational vehicles on traffic operations. PCEs are currently being used for studying freeways, multilane highways, and two-lane highways. A heavy-vehicle factor is directly given for the impact of heavy vehicles at signalized intersections (and indirectly along arterials). These PCE values are typically based on a limited number of simulations and on older simulation models. In addition, the impact of variables such as traffic flow, truck percentage, truck type (i.e., length and weight/horsepower ratio), grade, and length of grade on PCEs has not been evaluated in depth for all facility types. The methodology for developing PCEs for different truck types for the full range of traffic conditions on freeways, two-lane highways, and arterials is described. Given the scope of this research and the variability of traffic conditions to be examined, simulation was selected a...

Development of Passenger Car Equivalents for Freeways, Two-Lane Highways, and Arterials

The American Association of State Highway and Transportation Officials (AASHTO) is revising "A Policy on Geometric Design of Highways and Streets," also referred to as the "Green Book" in the transportation field. The "Green Book" is undergoing a conversion from English to metric units, which was necessitated by the need to comply with a federal law requiring that each federal government agency begin using the metric system in all procurements, grants, and other business-related activities. This article reviews the major revisions to the "Green Book" and summarizes the changes to various chapters.

L King

Papers

The new aashto metric policy on geometric design of highways and streets