Recent Roadway Geometric Design Research for Improved Safety and Operations (2012) / Chapter Skim
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Pages 58-64
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From page 58... ...
They posited that a potential source for an adjustment factor for entrance ramps is the calculation of the distance needed to accelerate from one speed to another on different grades by means of vehicle performance equations available in the literature, and thus they reviewed the literature to develop potential acceleration length adjustment factors. They subsequently applied the Green Book methodology for calculating SSD on different grades to the equations used to calculate deceleration lengths so as to determine deceleration lengths for different grades.
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From page 59... ...
Combination Ramp Spacing Dimension (ft) Feasibility Diamond Interchange Entrance–Exit Less than 1,600 Likely not geometrically feasible 1,600 to 2,600 Potentially geometrically feasible Greater than 2,600 Likely geometrically feasible Partial Cloverleaf Entrance–Exit Less than 1,600 Likely not geometrically feasible 1,600 to 1,800 Potentially geometrically feasible Greater than 1,800 Likely geometrically feasible Entrance–Entrance Less than 1,400 Likely not geometrically feasible 1,400 to 1,800 Potentially geometrically feasible Greater than 1,800 Likely geometrically feasible Exit–Exit Less than 900 Likely not geometrically feasible 900 to 1,100 Potentially geometrically feasible Greater than 1,100 Likely geometrically feasible Exit–Entrance (Braided)
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From page 60... ...
However, on the cross street, the traffic moves to the left side of the roadway between the ramp terminals. This allows the vehicles on TABlE 21 wEAVING DISTANCES FOR MANAGED lANE CROSS-FREEwAy MANEUVERS Design Year Volume Level Allow up to 10 mph Mainline Speed Reduction for Managed Lane Weaving Intermediate Ramp (between freeway entrance/exit and managed lanes entrance/exit)
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From page 61... ...
Notes: 4' T yp ica l Managed Lane General Purpose Lane 8" White Line 8" White Line Detail A 8" White Line 4' T yp ica l Detail C 3' 3' 12'12' Managed Lane General Purpose Lane 3' 12' 45' Raised Pavement Markers Type II-C-R 8" White Line Detail B 3' 3' 12'12' Managed Lane General Purpose Lane 3' 12' 45' Raised Pavement Markers Type II-C-R 3' See Detail B Raised Pavement Markers Type I-C or Type II-C-R 40' Max. - 20' Desirable1" Gap FIGURE 21 Design of intermediate at-grade access opening for buffer-separated freeway managed lane (Fitzpatrick et al.
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From page 62... ...
Displaced left-Turn The DlT interchange, also known as the continuous flow interchange, is an innovative interchange design that has several aspects similar to the at-grade DlT intersection and some aspects similar to the DCD interchange. It is a design treatment that has been advocated as promising because it removes the conflict at the main intersection between left-turning and opposing through vehicles (Hughes et al.
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From page 63... ...
(2010) cite research into the operation of DlT intersections sponsored by the Maryland State Highway Administration that revealed that the distance between the crossover and the main intersection was dependent on queuing from the main intersection and on costs involved in constructing a left-turn storage area.
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From page 64... ...
Geometric design principles, as well as site-specific features, dictate minimum lengths needed for ramps and other interchange components. Traffic volumes can necessitate increased spacing beyond the dimensions needed purely for geometrics.
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