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4.0 Geometric and Safety Design

4.2.1 State-of-the-Practice

Departure recovery zones from the project survey results range from 100 feet to over 1300 feet, with an average of 500.7 feet. Although it is uncertain whether the longer lengths include the transition areas, this is likely the case. As shown in Table 4.2, departure zone taper rates on toll roads were found to range from approximately 3 to 40, with an average of 9. For toll bridges, this range was from 4 to 11, with an average of 6. As expected, tapers for bridges and tunnels tend to be shorter, presumably the result of a shorter overall departure zone.

Table 4.2 Conventional Plaza Departure Taper Rates

Taper Rates

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

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