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72 Superelevation may not be needed because it could "train" drivers how to use the new intersections are also detract from any desired traffic calming effect. considerations. Lane width should be around 15 ft. · As roadway agencies continue to investigate new ways Design should accommodate WB-67 trucks. to use their budgets more efficiently, the cost of any Adequate lighting should be provided. treatment will likely be further scrutinized, whether it Nearside signals should be considered. is the realignment of a skewed intersection or the addi- Double Crossover Diamond interchange designs tion of rumble strips to a lengthy section of two-lane may only be appropriate where there are high-turning highway. Treatments that can provide benefits at low volumes. costs would appear to become increasingly valuable Nearby intersections with high cycle lengths should and desirable in this fiscal environment. be avoided. Pedestrians at free-turning movements should be Recommendations for Further Research evaluated, and pedestrian signals may be needed. The noses of the median island should extend beyond During the course of their projects, many researchers identi- the off-ramp terminals to improve channelization fied gaps in knowledge or additional questions that were raised and prevent erroneous maneuvers. as a result of their findings. Other needs for future research Left- and right-turn bays should be designed to allow have also been identified based on information that was not for separate signal phases." found within the literature that was reviewed for this synthesis · The Displaced Left-Turn interchange has functions report. Recommendations for research to fill those needs are similar to a DLT at-grade intersection. DLT lanes typi- summarized here: cally cross the opposing through traffic at locations that are approximately 400 to 500 ft upstream of the signal- · Fitzpatrick et al. (2006) recommended that their findings controlled ramp terminals. Minimum median widths on safety and operations at exclusive right-turn lanes be are preferred for this design (Hughes et al. 2010). verified through use of a larger, more comprehensive study that includes right-turning volume. Barriers to Widespread Implementation · Multiple studies mentioned the lack of data on U-turns at median openings not designed for U-turns and/or sug- This section discusses some potential barriers to the wide- gested this as a valid research topic to examine safety spread implementation of the research and findings presented and operational effects of such maneuvers. within the report. These potential barriers are presented as · Carter et al. (2005a) discussed several potential research observations gleaned through the compilation of the material topics for U-turns at signalized intersections. Among collected for this research. them are potential benefits of "U-turn Must Yield" signs; mitigation of the effects of right-turn overlap; · A large number of the sources reviewed for this synthesis a U-turn prediction model based on driveway density, produced results and recommendations that incorporated land usage, and other site characteristics; and the effects the use of a series of complex equations and/or multiple on capacity and safety of U-turning heavy vehicles. assumptions to begin the analysis. Such complex meth- · NCHRP Report 672 (Rodegerdts et al. 2010) advised odology may not be conducive to practitioners because the use of a critical headway of 5.0 s, based on the criti- the complex equations do not facilitate their use or cal headway required for passenger cars. The authors because the necessary data are not available. added that this value represented an interim methodol- · Similarly, the use of computer-based simulation and ogy pending further research. modeling has greatly increased as technology improves. · With the advent and increasing popularity of electronic However, many designers, particularly those at agen- toll collection methods, toll plaza design practices are cies in smaller jurisdictions, do not have access to such changing to a certain degree. Among the reviewed sources software or expertise to successfully use it to obtain the and the practitioners who focus on this area of geometric results described in the research. design, there appears to be a consensus that more recent · The advent of multiple innovative intersection treat- information on updated practices may be fragmented, ments has led to a wide variety of potential outcomes, scattered, or not yet evaluated; a need exists for at least and the research to support those outcomes is not yet a compendium of the best knowledge currently available mature. Practitioners who desire to use one or more on those measures found to be the most successful in the of these treatments are cautioned in multiple studies application of geometric criteria in the design of fixed- that results are still very preliminary. In addition, these barrier manually operated plazas as well as in the removal treatments typically require additional right-of-way and of the barriers and replacement with electronic open-road construction costs. Although they may be less expen- tolling gantries. sive than a fully grade-separated facility, the cost is still · Research is needed on intermediate speeds in the range a major factor in determining which treatment to use. of 40 to 50 mph in urban and suburban areas, and their The added complexity of the design and the need to effects on various cross-sectional design elements. Such
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73 cross-section elements include the allocation of lane ponents are simply put together in the final design of and shoulder widths, use of various median types and a ramp. Ramp design practices may consider driver widths, the provision of bike lanes, parking lanes, use expectations and behaviors over a full range of geomet- of vertical or sloping curbs and gutters and associated ric and traffic conditions that would include the inter- offsets, clear zone widths, traffic barriers, utilities, and change form, ramp type, the area environment (rural vs. interactions of various combinations of these elements. urban) and the functional classification of the two inter- · Highway designers are under increasing pressure to max- changing roadways. The issue of an integrated ramp imize the use of available right-of-way in freeway corri- and ramp terminal design is a complex issue in need of dors to provide safety, mobility, and capacity for growing basic research. traffic demand. With right-of-way limitations, increased · Decision sight distance policy is based on a relatively use of context-sensitive designs, and implementation of small research study completed for FHWA in 1978 managed facilities, designers must maximize the use of (McGee et al.). Decision sight distance is clearly intended freeway cross sections. Although freeway cross-section for application at selected locations where greater sight design guidance suggests that 12-ft lanes with 8- to 10-ft distance than SSD is needed. However, there is little inside and outside shoulders is ideal, there is limited practical guidance to help designers identify situations research on how deviations from these ideals individu- where decision sight distance is or is not appropriate. ally, or in combination, will affect freeway operations and And, there is little available information on how decision safety. Highway designers need guidance on the opera- sight distance criteria are actually being applied by high- tional and safety impacts for cross-section design trade- way engineers and whether the decision sight distance offs while trying to balance corridor capacity, project policy is accomplishing its stated objective. costs, public involvement, and environmental impacts. · Traffic calming guidelines often discuss the benefits of · In addition, there is concern over the part-time use of designing roadways to improve pedestrian safety. In the- existing shoulders as high-occupancy vehicle, high- ory, roadways that are designed with certain characteris- occupancy toll, or general-use facilities during peak hour. tics can encourage slower motor vehicle speeds, which The trade-offs between operational benefits and safety cause more motor vehicle drivers to yield to pedestrians need to be quantified. Further, the safety implications of crossing the street and result in less severe pedestrian violators using the shoulder during the off-peak period injuries when crashes do occur. Yet, there is a lack of need to be quantified. It is unclear whether this changed research that quantifies the complexity of relationship view of the shoulder as part of the traveled way also between the following three factors: (1) roadway design, transfers to shoulder violations on adjacent facilities. (2) motor vehicle speed, and (3) motorist yielding The signing and striping of these shoulders for clear behavior. The effects of roadway design treatments on communication of the changed cross section use must driver yielding are unknown for many different combi- also be quantified. nations of traffic speed and roadway conditions. This · Despite the many features of the Interactive Highway makes it extremely difficult to craft pedestrian-oriented Safety Design Model, methods to assess design con- guidelines that are applicable to the wide range of con- sistency for multi-lane rural highways and urban and ditions present in communities throughout the country. suburban arterials are not available. Development of More research is needed to quantify how driver yielding design consistency procedures for these facility types behavior is related to travel speed and different roadway will provide a full suite of mobility and safety assess- characteristics, such as lane widths, pavement condi- ment tools for use by designers throughout the project tions, horizontal and vertical shifts, sight distances, development process. lateral clearance, and other factors. This research should · Typically, ramp terminals and the ramp proper are be used to create improved guidelines for roadway designed independent of each other and the two com- design and traffic calming practice.