Quantcast Transitions and Tapers

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4.2.4.4
Curbs and Gutters/Shoulders
Shoulders are desirable for accommodation of disabled or stopped vehicles (7, 20).
However, curb and gutter are preferable in areas where lane control is desired and to
improve safety. Therefore, shoulders are recommended for use in the approach and response
zones. Curb and gutter is required in the access control zone and around all channelization
islands.
4.2.4.5
Transitions and Tapers
When lanes are redirected, dropped, or added, proper transitions must be provided to
enhance traffic flow and ensure safety (2). The transition affords drivers the opportunity to
recognize the change and react appropriately (2). At a minimum, transitions will be required
to redirect lanes at the curbed islands. Ref. 2 provides transition length criteria for the
situations listed above. Other traffic engineering references will provide useful information
and design guidance for these situations as well.
The recommended transition distances in Ref. 2, should be followed as minimums unless it can
be shown through proper traffic engineering that shorter transitions will not inhibit the flow of
traffic through the entry control facility. The minimum speed (S) used for determining transition
lengths will be 40 kmph (25 mph). If the minimum transition distances are unavailable, then
the lane addition, lane deletion, or lane width modification requiring the transition shall be
eliminated.
4.2.4.6
Roadside Safety
Barrier end treatments and the need for crash cushions will be determined in accordance with
AASHTO Roadside Design Guide, latest edition. For personnel protection requirements see
Section 4.11.
4.2.4.7
Lateral Clearances
Lateral obstructions present a safety hazard and tend to negatively impact traffic flow. The
negative effects are eliminated or reduced if the object is less than 152 mm (6 in.) in height or
located at least 1.8 meters (6 ft.) from the edge of the roadway (9, 247). Ideally the lateral
clearance would be greater. Therefore, the location of lateral obstructions in the approach
and response zones, including the passive vehicle barriers, shall be a minimum of 1.8 meters
(6 ft.) from the edge of roadway. It is recommended that passive vehicle barriers defining
the boundary of the ECF be located 3.6 meters (12 ft) from the edge of road. Where
passive vehicle barriers must extend close to the active vehicle barriers at the end of the
response zone to maintain containment, this minimum is not applicable. Additionally, in the
access control zone the location of facilities and access control equipment will likely provide
less lateral clearance. Where possible, a minimum lateral clearance of 610 mm (2 ft.) should
be maintained in the access control zone to allow security personnel to pass between the
obstruction and the roadway.
4.2.4.8
Vertical Clearances
AASHTO standards indicate a minimum vertical clearance of 4.3 meters (14 ft.) for highways
or 4.9 meters (16 ft.) for freeways. In order to support potential over height vehicles and/or
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