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4.0 Geometric and Safety Design

4.1.1 State-of-the-Practice

Approach queue zones from the project survey results range from 100 feet to over 1300 feet, with an average of 528.7 feet. Lengths may be attributed to an inadequate number of toll plaza lanes or the longer lengths could include the transition areas. Taper rates on toll roads were found to range from approximately 4 to 40, with an average of 9. On toll bridges, this range was from 2 to 17, with an average of 5. As expected, tapers for bridges and tunnels tend to be shorter, presumably the result of a shorter overall approach zone.

Table 4.1 Conventional Plaza Approach Taper Rates

Taper Rates

The MUTCD 2003 and AASHTO “Green Book” provide diverging taper rates for lane additions and drops. However, neither of these sources provides taper rates specifically for the approach zones of conventional plazas. The AASHTO “Green Book’s” policy of interdependence of taper rates and speed should be considered in the design to assure a safe transition.

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