The results of research involving a well-designed before-and-after evaluation of the safety effects of providing left- and right-turn lanes for at-grade intersections are presented. Geometric design, traffic control, traffic volume, and traffic accident data were gathered for a total of 280 improved intersections as well as 300 similar intersections that were not improved during the study period. The types of improvement projects evaluated included installation of added left-turn lanes, added right-turn lanes, and extension of the length of existing left- or right-turn lanes. An observational before-and-after evaluation of these projects was performed by using several alternative evaluation approaches. Three contrasting approaches to before-and-after evaluation were used: the yoked comparison or matched-pair approach, the comparison group approach, and the empirical Bayes approach. The research not only evaluated the safety effectiveness of left- and right-turn lane improvements but also compared the performances of these three alternative approaches in making such evaluations. The research developed quantitative safety effectiveness measures for installation design improvements involving added left-turn lanes and added right-turn lanes. The research concluded that the empirical Bayes method provides the most accurate and reliable results. Further use of this method is recommended.

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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 as the most appropriate tool. The resulting PCE values for freeways, two-lane highways, and arterials indicated that some variables, such as percentage of trucks, do not always have the expected effect on PCEs, whereas other variables, such as vehicle type, are crucial in the calculations. Generally, major differences in PCEs occurred for the longer and steeper grades. There was great variability in PCE values as a function of the weight/horsepower ratio as well as of vehicle length.Passenger car equivalents (PCEs) have been used extensively in the Highway Capacity Manual (1,2) to establish the impact of trucks, buses, and recreational vehicles (RVs) on traffic operations. PCEs are currently being used for freeways, multilane highways, and twolane highways. PCE values for freeways and multilane highways are given as a function of grade, length of grade, and percentage of trucks, buses, or RVs. For two-lane highways, PCEs are given as a function of the type of terrain and level of service or average upgrade speed for trucks, buses, and RVs. The impact of heavy vehicles at signalized intersections (and indirectly along arterials) is given as a function of the percentage of heavy vehicles. The impact of grade is considered independently of heavy-vehicle presence for two-lane highways and for signalized intersections.These PCE values are typically based on small databases and a limited number of simulations with older models (3). 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 as the most appropriate tool. It must be noted that the research described here was performed as p...

This paper presents the results of a study to develop improved design guidance for freeway main-line ramp terminals on the basis of modern driver behavior and vehicle performance capabilities. The primary steps included a crash analysis, an observational field study, and a driver behavioral study. This research reached several key conclusions, as follows. Vehicle merging speeds tend to be closer to freeway speeds at tapered acceleration lanes than at parallel acceleration lanes. The recommended minimum lengths for acceleration lanes presented in the 2004 edition of AASHTO's A Policy on Geometric Design of Highways and Streets (Green Book) are conservative and, under certain conditions, could be reduced by 15%. In a situation in which a significant volume of trucks uses an entrance ramp, an acceleration lane length that will better accommodate trucks can be derived by using speed–distance curves developed for a range of weight-to-power ratios. The recommended minimum lengths for deceleration lanes presented in the 2004 Green Book are conservative and do not account for deceleration in the freeway. Providing deceleration lanes longer than the minimum values given in the Green Book may promote casual deceleration by exiting drivers. Several potential changes for consideration in the next edition of the Green Book are recommended on the basis of the findings and conclusions of this research.

Systematic, well-designed research provides the most effective approach to the solution of many problems facing highway administrators and engineers. Often, highway problems are of local interest and can best be studied by highway departments individually or in cooperation with their state universities and others. However, the accelerating growth of highway transportation develops increasingly complex problems of wide interest to highway authorities. These problems are best studied through a coordinated program of cooperative research. In recognition of these needs, the highway administrators of the American Association of State Highway and Transportation Officials initiated in 1962 an objective national highway research program employing modern scientific techniques. This program is supported on a continuing basis by funds from participating member states of the Association and it receives the full cooperation and support of the Federal Highway Administration, United States Department of Transportation. The Transportation Research Board of the National Academies was requested by the Association to administer the research program because of the Board's recognized objectivity and understanding of modern research practices. The Board is uniquely suited for this purpose as it maintains an extensive committee structure from which authorities on any highway transportation subject may be drawn; it possesses avenues of communications and cooperation with federal, state and local governmental agencies, universities, and industry; its relationship to the National Research Council is an insurance of objectivity; it maintains a full-time research correlation staff of specialists in highway transportation matters to bring the findings of research directly to those who are in a position to use them. The program is developed on the basis of research needs identified by chief administrators of the highway and transportation departments and by committees of AASHTO. Each year, specific areas of research needs to be included in the program are proposed to the National Research Council and the Board by the American Association of State Highway and Transportation Officials. Research projects to fulfill these needs are defined by the Board, and qualified research agencies are selected from those that have submitted proposals. Administration and surveillance of research contracts are the responsibilities of the National Research Council and the Transportation Research Board. The needs for highway research are many, and the National Cooperative Highway Research Program can make significant contributions to the solution of highway transportation problems of mutual concern to many responsible groups. The program, however, is intended to complement rather than to substitute for or duplicate other highway research programs.