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59 chapter six Interchanges Overview and on a level surface should be measured and compared with the suggested adjustment factors to determine the accu- Similar to intersections, new ways to design interchanges racy of those factors. received attention during the last 10 years in an attempt to improve capacity while minimizing the cost of constructing or expanding the interchange. Researchers also revisited charac- Ramp and Interchange Spacing teristics of ramp design and ramp terminal design, and they considered the effects of work zones near interchanges. Under NCHRP Project 03-88, researchers evaluated and summarized design, operations, safety, and signing consid- erations that influence ramp and interchange spacing deci- Design of Ramps and Ramp Terminals sions (Ray et al. 2011). The Green Book contains guidelines on the distance between successive ramp terminals, but Chaudhary and Messer (2002) developed guidelines for design- they "are acknowledged to be based on operational experi- ing freeway on-ramps in which ramp metering is envisioned. ence and recommend basing actual spacing on operations Specifically, they looked for design issues in which ramp and safety procedures derived from applied research." To meters use a queue detector to identify and prevent a queue provide a better understanding of the impacts of ramp and of vehicles from blocking the upstream intersection. They interchange spacing on safety and operations, research- focused on three design elements: safe stopping distance, ers collected and analyzed data from a variety of existing storage distance, and acceleration distance from meter to freeway ramps and interchanges, focused on relatively simple, merge point. Combining these three elements, they stated single lane, service ramps and interchanges. The team con- that the desired distance between the cross street and freeway ducted operational and safety assessments of two types of merge point be at least 400 m (1,312 ft) for ramps at which ramp pairs--an entry ramp followed by an exit ramp (EN-EX) metering is envisioned. and an entry ramp followed by another entry ramp (EN-EN). They then performed simulation modeling, calibrated with Fitzpatrick and Zimmerman (2007) reviewed the Green field data, of closely spaced pairs of ramps and developed Book's process for adjusting acceleration and deceleration safety performance models. lengths on graded ramps. They found that the source of the adjustment factors in the 2004 Green Book was provided in Based on their findings, the team then developed guide- the 1954 Policies on Geometric Highway Design (i.e., the lines to assist practitioners in selecting ramp and interchange Blue Book, AASHO 1954), in which they first appeared as spacing values for their particular design context. These guide- being based on applying "principles of mechanics to rates lines presented substantial discussions on geometric design, of speed change for level grades." Their reviews of that traffic operations, safety, and signing, and the role each of document and others did not reveal a procedure for deter- these play in determining ramp and interchange spacing mining adjustment factors. They posited that a potential needs. The guidelines made a distinction to separately define source for an adjustment factor for entrance ramps is the "ramp spacing" and "interchange spacing" and recommended calculation of the distance needed to accelerate from one ramp spacing values be the primary consideration in freeway speed to another on different grades by means of vehicle and interchange planning and design. Guidelines were pre- performance equations available in the literature, and thus sented based on four areas of emphasis: geometric design, they reviewed the literature to develop potential accelera- traffic operations, signing, and safety. Geometric design tion length adjustment factors. They subsequently applied principles, as well as site-specific features, dictate minimum the Green Book methodology for calculating SSD on dif- lengths needed for ramps and other interchange compo- ferent grades to the equations used to calculate decelera- nents. Traffic volumes can necessitate increased spacing tion lengths so as to determine deceleration lengths for beyond the dimensions needed purely for geometrics. Safety different grades. The ratio of the deceleration length on a tradeoffs, which have rarely been quantified until recently, grade to the deceleration length on a level surface formed can now be considered in project decision making. Finally, the basis for their adjustment factors for deceleration. They signing and other human factors issues should be taken recommended that actual performance of vehicles on grades into account at the earliest in the evaluation process when

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60 Table 20 Potential Feasibility of Spacing For Various Freeway Ramp Combinations Ramp Spacing Combination Dimension (ft) Feasibility Diamond Interchange Less than 1,600 Likely not geometrically feasible EntranceExit 1,600 to 2,600 Potentially geometrically feasible Greater than 2,600 Likely geometrically feasible Partial Cloverleaf Less than 1,600 Likely not geometrically feasible EntranceExit 1,600 to 1,800 Potentially geometrically feasible Greater than 1,800 Likely geometrically feasible EntranceEntrance Less than 1,400 Likely not geometrically feasible 1,400 to 1,800 Potentially geometrically feasible Greater than 1,800 Likely geometrically feasible ExitExit Less than 900 Likely not geometrically feasible 900 to 1,100 Potentially geometrically feasible Greater than 1,100 Likely geometrically feasible ExitEntrance Less than 1,700 Likely not geometrically feasible (Braided) 1,700 to 2,300 Potentially geometrically feasible Greater than 2,300 Likely geometrically feasible Source: Ray et al. (2011). making choices about ramp and interchange spacing. The scenarios. The researchers developed tables containing the guidelines were presented as information that can also be cost data that planners, designers, and policymakers could incorporated in future editions or updates of relevant man- use in determining their choice of intersection and access uals and other guidance documents. Among the geometric spacing for specific freeway ramp locations. design guidelines are spacing assessments, as shown in Table 20. Managed Lanes Access Management Fitzpatrick et al. (2003b) conducted an evaluation of managed lane ramp design issues in Texas, with a comparison to then- The adequate spacing and design of access to crossroads in the current practices in national and other states' guidelines. vicinity of freeway ramps are critical to the safety and traffic The 2001 Green Book (AASHTO 2001) specified a 2,000-ft operations of both the freeway and the crossroad. Rakha et al weaving section for a system-to-service interchange. For a (2008) conducted research for the Virginia DOT to develop direct-connection ramp between a traffic generator and the a methodology to evaluate the safety impact of different managed lane, AASHTO recommended a minimum design access road spacing standards. The models they developed speed for direct connection ramps of 40 mph, whereas Cali- were used to compute the crash rate associated with alter- fornia's guidelines (Caltrans 2001) called for a minimum native section spacing, and the authors concluded that the of 50 mph. Each state's guidelines that contained specific models satisfied statistical requirements and provided rea- discussions on the spacing between successive ramps used sonable crash estimates. Their results indicated that the crash approximately 900 to 1,000 ft spacing. rate decreased by 88% when access road spacing increased from 0 to 300 m. An increase in the minimum spacing from They also used computer simulation "to obtain an appre- 90 m (300 ft) to 180 m (600 ft) resulted in a 50% reduction ciation of the effects on corridor operations when several in the crash rate. The models were used to develop lookup pairs of ramps are considered. Speed was the primary mea- tables that quantified the impact of access road spacing on sure of effectiveness used to evaluate the effects of different the expected number of crashes per unit distance. Those ramp spacings, volume levels, and weaving percentages. The tables revealed a decrease in the crash rate as the access research found that a direct connect ramp between a gen- road spacing increases. The researchers also attempted to erator and the managed lane facility should be considered quantify the safety cost of alternative access road spacing when 400 veh/hr is anticipated to access the managed lanes. using a weighted average crash cost. The weighted average If a more conservative approach to preserving freeway per- crash cost was computed based on the observed distribution formance is desired, then a direct connect ramp should be of crashes in Virginia that were fatal, injury, and property considered at 275 veh/hr (which reflected the value when damage crashes. Costs of crashes in each severity category the lowest speeds on the simulated corridor for the scenarios were provided by the Virginia DOT, which the researchers examined were at 45 mph or less)." This finding builds on the used to compute an average weighted crash cost. This aver- recommendations made by Venglar et al. (2002) on weaving age cost was multiplied by the number of crashes per mile distances for managed lane cross-freeway maneuvers, shown to compute the cost associated with different access spacing in Table 21.