Click for next page ( 64

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 63
64 lane of approximately 4.5 m (15 ft), with a 1.8-m (6-ft) right shoulder and minimum 0.6-m (2-ft) left shoulder. However, different cross-sectional arrangements are appropriate when supported by agency experience and in consideration of project-specific factors (e.g., traffic volume, mix, and duration of service). FIGURE 23 Detailed view of movements and paths for half of a With respect to entrance ramps, they concluded that the fea- DLT interchange. sibility of maintaining an entrance during construction often hinges on providing an adequate combination of roadway geometry and traffic control to facilitate merging. Figure 24 make the left turn onto the ramp from the new roadway after illustrates a temporary entrance ramp for a median crossover. crossing over the freeway, as shown at the top of Figure 23. The authors concluded that the basic principles and issues associated with permanent ramps also pertain to temporary As with a DLT intersection, the differentiating design ele- arrangements. Therefore, acceleration lanes in work zones ment of a DLT interchange is the left-turn crossover. The DLT that meet the design criteria for permanent facilities were lanes typically cross the opposing through traffic at locations desirable. However, providing these lane lengths was often that are approximately 400 to 500 ft upstream of the signal- not practical. Thus, they provided several "rules of thumb" as controlled ramp terminals. Geometrically, the left-turn cross- options to guide designers: over in a DLT interchange is similar to the design of a left-turn crossover for a DLT intersection. Hughes et al. (2010) cite Provide at least 90 m [300 ft] of acceleration lane. research into the operation of DLT intersections sponsored by Provide at least 70% of the permanent roadway criteria the Maryland State Highway Administration that revealed that the distance between the crossover and the main intersection length. was dependent on queuing from the main intersection and on Employ traffic control measures (e.g., STOP, YIELD, costs involved in constructing a left-turn storage area. Radii of and other signs) to mitigate less-than-desirable accel- the crossover movements range from 150 to 200 ft. The radii eration lane lengths. of the left-turn movement at the nodes of the interchange are dependent on the turning movement of a design vehicle. The authors also offered similar guidance for exit ramps, stating that it was desirable for exiting traffic to depart the Median width affects the interchange footprint and con- through lanes at mainline speed and not reduce speed while sequently the right-of-way acquisition. As with DLT inter- occupying the mainline through lane. When this is not prac- sections, FHWA encourages designers to obtain minimum tical, they recommended that the geometry of the ramp be median widths from the AASHTO Green Book; offset rec- reviewed to determine if the ramp's length, horizontal align- ommendations for post-mounted signs should be accounted ment, and grade allow for gradual deceleration before reaching for in accordance with MUTCD when determining median speed-critical features. width, though the minimum median width for any type of intersection or interchange is 4 ft (Hughes et al. 2010). The Summary of Key Findings authors further state that a wide median is counterproductive at a DLT interchange for the following reasons: This section summarizes key findings from the research noted in this chapter. This is an annotated summary; conclu- Wide medians result in long walking distances for pedes- sions and recommendations are those of the authors of the trians at the interchange. In turn, this results in the need references cited. for long pedestrian clearance intervals and potentially increased cycle lengths, which is counterproductive to Interchange Ramp Design traffic efficiency. Wide medians necessitate a wide interchange footprint The desired distance between the cross street and freeway and consequently higher bridge deck construction costs. merge point is at least 400 m (1,312 ft) for ramps at which metering is envisioned (Chaudhary and Messer 2002). Work Zone Considerations The source of the adjustment factors in the 2004 Green Book was provided in the 1954 AASHTO Blue Book, NCHRP Report 581 (Mahoney et al. 2007) discusses a variety in which they first appeared as being based on apply- of geometric design principles and their applications within ing "principles of mechanics to rates of speed change work zone traffic control on high-speed highways. In partic- for level grades." Further review did not reveal a pro- ular, the authors discussed the appropriate design principles cedure for determining adjustment factors. A new pro- for temporary entrance and exit ramps. They stated that a cedure contains an alternative set of adjustment factors temporary single-lane interchange ramp should have a travel for acceleration length and deceleration length, the latter

OCR for page 63
65 FIGURE 24 Temporary interchange entrance ramp for median crossover (Mahoney et al. 2007). of which is based on the ratio of the deceleration length Alternative Interchange Designs on a grade to the deceleration length on a level surface. Actual performance of vehicles on grades and on a level Design practices for the DDI (Hughes et al. 2010) surface should be measured and compared with the sug- include: gested adjustment factors to determine the accuracy of The minimum crossing angle of intersection should those factors (Fitzpatrick and Zimmerman 2007). be 40 degrees. The radius design should accommodate between 25 and 30 mph. Ramp and Interchange Spacing Superelevation may not be needed because it could Recent guidelines make a distinction to separately define detract from any desired traffic calming effect. "ramp spacing" and "interchange spacing" and recom- Lane width should be approximately 15 ft. mend ramp spacing values be the primary consideration Design should accommodate WB-67 trucks. in freeway and interchange planning and design (Ray Adequate lighting should be provided. et al. 2011). Nearside signals should be considered. Guidelines are presented based on four areas of emphasis: DCD interchange designs may only be appropriate geometric design, traffic operations, signing, and safety. where there are high-turning volumes. Geometric design principles, as well as site-specific Nearby intersections with high cycle lengths should features, dictate minimum lengths needed for ramps be avoided. and other interchange components. Traffic volumes can Pedestrians at free-turning movements should be necessitate increased spacing beyond the dimensions evaluated, and pedestrian signals may be needed. needed purely for geometrics. Safety tradeoffs, which The noses of the median island should extend beyond until recently have rarely been quantified, can now be the off-ramp terminals to improve channelization considered in project decision making. Finally, signing and prevent erroneous maneuvers. and other human factors considerations should be taken Left- and right-turn bays should be designed to allow into account at the earliest in the evaluation process when for separate signal phases. making choices about ramp and interchange spacing (Ray The Displaced Left-Turn interchange has functions et al. 2011). similar to a DLT at-grade intersection. DLT lanes typi- Spacing assessments indicate that ramp spacing of less cally cross the opposing through traffic at locations that than 900 ft is likely not geometrically feasible. That are approximately 400 to 500 ft upstream of the signal- spacing value increases up to 1,600 ft for entranceexit controlled ramp terminals. Minimum median widths ramp pairs (Ray et al. 2011). are preferred for this design (Hughes et al. 2010).