Access Management in the Vicinity of Interchanges, Volume 2: Research Overview (2022) / Chapter Skim
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4 State of Practice and Literature Review The spacing of crossroads in the vicinity of interchanges can have operational and safety impacts on a street system. The deployment of access management strategies in these areas can also have potential influence on the economic vitality of a roadway network, but how best to balance these critical issues with access management strategies in not yet fully understood.
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State of Practice and Literature Review 5 Subsequent to the NCHRP Synthesis 332 effort, a few studies have addressed access control on crossroads for specific states. In 2006, researchers in Florida evaluated driveway and median configurations in the vicinity of interchanges (10)
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6 Access Management in the Vicinity of Interchanges W DSD + Queue W DSD + Queue Source: AMAG (4)
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State of Practice and Literature Review 7 The following schematics represent potential terminal access configurations for these free-flow ramp terminal exits. The free-flow ramp terminal spacing scenarios include • Spacing to the first driveway (Figure 6)
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8 Access Management in the Vicinity of Interchanges W DSD + Queue DSD + Queue W Source: AMAG (4)
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State of Practice and Literature Review 9 DSD to S/P/∆ or DSD + Queue Z DSD to S/P/∆ or DSD + Queue Z Source: AMAG (4)
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10 Access Management in the Vicinity of Interchanges State RIRO Access(ft) Full Access -- Undivided (ft)
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State of Practice and Literature Review 11 The four categories specifically shown in Table 1 can be expanded to also include right-in, rightout, left-in (RIROLI) driveways (located at directional median openings)
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12 Access Management in the Vicinity of Interchanges As depicted in the AMAG, Table 3 and Figure 11 collectively provide spacing values for area type and road width at signalized ramp terminals. In contrast, Table 4 and Figure 12 depict potential spacing values for free-flow ramp terminals.
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Source: AMAG (4)
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14 Access Management in the Vicinity of Interchanges Type of Area Arterial Width Design Speed (mph) Spacing Dimension (ft)
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State of Practice and Literature Review 15 The AMAG compiled information from a variety of these comprehensive references to develop typical spacing proposals for signalized ramp terminals with two- and four-lane crossroads (see Table 3 and Figure 11)
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16 Access Management in the Vicinity of Interchanges Upon inspection, it is evident that these 15 factors are not independent; however, to fully understand the individual influence and potential interaction a factor may have with other road characteristics, it is important to review each factor individually. Each of these factors is therefore briefly discussed in the following sections, along with related research in the topical areas.
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State of Practice and Literature Review 17 • Partial Cloverleaf (PARCLO) , • Full Cloverleaf, and • All Directional Four Leg.
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18 Access Management in the Vicinity of Interchanges It is important to note that, although some agencies have well-established design standards and regulations for access spacing on crossroads, the overall effectiveness of these access management strategies is significantly affected by the prior planning and the design used to implement a retrofit or new interchange project. The decision regarding the appropriate interchange form should incorporate the needs of the surrounding transportation system and land use to minimize the number of weaving movements and vehicular conflicts necessary to access the road system and adjacent property.
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State of Practice and Literature Review 19 requirements from the distance needed to traverse from the interchange terminal to the appropriate lane before vehicles encounter the back of queues created by the downstream, signalized intersection. Because this process is highly dependent on the knowledge of future traffic demands and land-use development, relatively few agencies use downstream storage requirements as an overall determining factor in establishing crossroad spacing requirements.
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20 Access Management in the Vicinity of Interchanges factors. The guiding principle behind this is that higher-volume roadways have the potential for more operational deficiencies and turning movement conflicts.
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State of Practice and Literature Review 21 Safety of Access Spacing There is very little research available regarding the safety implications of access management in the vicinity of interchanges. A Virginia study (12)
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