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Lateral Clearances

Bridge railings located on the inside of horizontal curves may restrict stopping sight distance. Adjustment of the horizontal alignment or the offset to the bridge railing may be needed to provide adequate stopping sight distance.

On overpass structures, it is desirable to carry the full width of the approach roadway across all structures. For facilities other than freeways, exception may be made on major structures with a high unit cost. The selection of cross-section dimensions that are different from those on the approach roadway should be subject to individual economic studies. Refer to previous chapters on arterials, collectors, and local roads and streets for permissible deviations from providing full approach roadway width across bridges. In the case of a curbed roadway, the minimum structure width should match the curbed approach roadway.

When the full approach roadway width is continued across the structure, the parapet rail, both left and right, should align with the guardrail on the approach roadway. For example, where the typical practice of the highway agency is to place the longitudinal barrier 0.6 m [2 ft] from the outer edge of the surfaced shoulder, the bridge rail should be placed 0.6 m [2 ft] outside the effective edge of the shoulder. This provides additional clearance for high-speed operation and door-opening space for vehicles stopped on the shoulder of the structure. Some agencies prefer to place the roadway longitudinal barrier 0.6 m [2 ft] from the outer edge of the shoulder and the bridge rail at the shoulder edge. In this case, a transition rate of about 20:1 is appropriate to taper the longitudinal barrier into the bridge rail.

At some interchanges, additional width for speed-change lanes or weaving sections is needed across overpass structures. Where the auxiliary lane is a continuation of a ramp, the horizontal clearance to the bridge rail should be at least equal to the width of shoulder on the approach ramp. Where the auxiliary lane is a weaving lane connecting entrance and exit ramps or is a parallel-type speed-change lane across the entire structure, the clearance to the parapet should be of uniform width and be at least equal to the shoulder width on the ramp.

Medians

On a divided highway with a wide median or one being developed in stages, the overpass will likely be built as two parallel structures. The approach width of each roadway should be carried across each individual structure. If separate parallel structures are used, the width of opening between structures is unimportant.

Where the approach is a multilane, undivided roadway or one with a flush median less than 1.2-m [4-ft] wide, a raised median is considered unnecessary on short bridges of about 30 m [100 ft] in length but is desirable on bridges of 120 m [400 ft] or more in length. On bridges between 30 m [100 ft] and 120 m [400 ft] in length, local conditions such as traffic volume, speed, sight distance, need for luminaire supports, future improvement, approach cross section,

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Exhibit 10-52. Coordination of Lane Balance and Basic Number of Lanes Through Application of Auxiliary Lanes

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Vertical Clearance

As discussed in earlier chapters, the shoulder on high-speed highways should be flush with the traveled way. Continuous curbs on high-speed highways should be limited to special situations, such as drainage systems on the outside of shoulders. Such curbs should be carried through the underpass. Where walkways are provided, the full shoulder section should be maintained through the underpass and the span increased by the width of the walk. Where a curb is needed along solid abutments or walls, a concrete barrier may be used. See Chapter 4 for a discussion on other types of barriers, their warrants, and issues concerning their placement.

Where conditions preclude the clear roadside design concept, all abutments, piers, and columns should be shielded with suitable protective devices unless they are so situated that they cannot be hit by out-of-control vehicles. Protective devices are usually not needed along continuously walled sections.

Guardrail installed along the face of an exposed pier or abutment should have a clearance appropriate to the dynamic lateral deflection of the particular rail type. The rail cannot cushion and deflect an errant vehicle unless there is sufficient lateral space clear of the bridge support. Exhibit 10-6C shows the limits of the dynamic lateral deflection distance between the face of bridge support and the back of the rail system. Guardrail attached flush with the exposed faces of piers, abutments, and bridge railings should be stiffened preceding the obstruction to avoid snagging an errant vehicle. This may be accomplished by reducing the post spacing, increasing the post embedment, increasing the rail section modulus, and/or transitioning to a different, stiffer barrier (i.e., metal to concrete). The rail should be fastened securely enough to develop its full strength longitudinally. For further details, see the AASHTO Roadside Design Guide (3).

Where structural design and cost features make it necessary to reduce the horizontal clearance through an underpass, the change in lateral width should be accomplished through gradual adjustments in the cross section of the approach roadway rather than abruptly at the structure. Such transitions in width should have a gradual rate of 50:1 or more (longitudinal:lateral).

Vertical Clearance

Vertical clearance is typically determined for an entire route and may be governed by the established policies of the highway system. Although State laws vary somewhat, most States permit the vehicle height, including load, to be between 4.1 m [13.5 ft] and 4.4 m [14.5 ft]. The vertical clearance of all structures above the traveled way and shoulders should be at least 0.3 m [1 ft] greater than the legal vehicle height, and allowance should be made for future resurfacing.

Additional vertical clearance is desirable to compensate for several resurfacings, for snow or ice accumulation, and for an occasional slightly overweight load. The recommended minimum vertical clearance is 4.4 m [14.5 ft], and the desirable vertical clearance is 5.0 m [16.5 ft].

Some roadways are parts of systems or routes for which a minimum vertical clearance of 4.9 m [16 ft], plus an allowance for future resurfacing, has been established. Freeway and arterial

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systems are generally provided with such clearance, but for other routes a lower minimum vertical clearance is acceptable.

To permit the movement of exceptionally high loads through an urban area, it is desirable to have at least one route with structures designed so that the movement can be easily accommodated. This design could entail the use of deck-type bridges, street lights mounted higher than normal, underground utilities, mast-arm-supported traffic signals, which can be swung to one side, etc.

Where a depressed facility is a parkway with traffic restricted to passenger vehicles, the vertical clearance at structures should be 4.6 m [15 ft], and in no case should it be less than 3.8 m [12.5 ft]. The minimum clearance should be obtained within all portions of the roadway.

Overpass Roadways

The roadway dimensional design of an overpass or other bridge should be the same as that of the basic roadway. The bridge is a small part of the continuous roadway and should be designed without change in cross-section dimensions, unless the cost becomes prohibitive.

This section covers the general dimensional features for single structures typically used at a grade separation, a stream crossing, or a single-structure interchange. Overpasses usually are deck structures. Their major dimensional features are the parapet rail system, lateral clearances, and the median treatment, where applicable. Typical overpass structures are shown in Exhibit 10-7. For further discussion see also the sections under "Curbs" and "Traffic Barriers" in Chapter 4.

Bridge Railings

The typical bridge railing has some form of concrete base or parapet on which metal or concrete rail or rails are mounted on structurally adequate posts. The bridge railing should be designed to accommodate the design vehicle(s) on the structure under the design impact conditions. That is, the design vehicle should be safely redirected, without penetration or vaulting over the railing. Likewise, the railing should not pocket or snag the design vehicle, causing abrupt deceleration or spinout, and it should not cause the design vehicle to roll over.

Most bridge railings in service are of a rigid, non-yielding design. Several railings incorporate energy-absorbing features in their design to reduce vehicle impact severity. Where noise is a factor, solid rails may be considered for their added value in noise attenuation.

At certain locations, there may be a need to provide a pedestrian walkway or bicycle path on the freeway overpass. In these situations, a barrier-type bridge rail of adequate height should be installed between the pedestrian walkway and the roadway. Also, a pedestrian rail or screen should be provided on the outer edge of the walkway.

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Exhibit 10-7. Typical Overpass Structures

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Lateral Clearances

Bridge railings located on the inside of horizontal curves may restrict stopping sight distance. Adjustment of the horizontal alignment or the offset to the bridge railing may be needed to provide adequate stopping sight distance.

On overpass structures, it is desirable to carry the full width of the approach roadway across all structures. For facilities other than freeways, exception may be made on major structures with a high unit cost. The selection of cross-section dimensions that are different from those on the approach roadway should be subject to individual economic studies. Refer to previous chapters on arterials, collectors, and local roads and streets for permissible deviations from providing full approach roadway width across bridges. In the case of a curbed roadway, the minimum structure width should match the curbed approach roadway.

When the full approach roadway width is continued across the structure, the parapet rail, both left and right, should align with the guardrail on the approach roadway. For example, where the typical practice of the highway agency is to place the longitudinal barrier 0.6 m [2 ft] from the outer edge of the surfaced shoulder, the bridge rail should be placed 0.6 m [2 ft] outside the effective edge of the shoulder. This provides additional clearance for high-speed operation and door-opening space for vehicles stopped on the shoulder of the structure. Some agencies prefer to place the roadway longitudinal barrier 0.6 m [2 ft] from the outer edge of the shoulder and the bridge rail at the shoulder edge. In this case, a transition rate of about 20:1 is appropriate to taper the longitudinal barrier into the bridge rail.

At some interchanges, additional width for speed-change lanes or weaving sections is needed across overpass structures. Where the auxiliary lane is a continuation of a ramp, the horizontal clearance to the bridge rail should be at least equal to the width of shoulder on the approach ramp. Where the auxiliary lane is a weaving lane connecting entrance and exit ramps or is a parallel-type speed-change lane across the entire structure, the clearance to the parapet should be of uniform width and be at least equal to the shoulder width on the ramp.

Medians

On a divided highway with a wide median or one being developed in stages, the overpass will likely be built as two parallel structures. The approach width of each roadway should be carried across each individual structure. If separate parallel structures are used, the width of opening between structures is unimportant.

Where the approach is a multilane, undivided roadway or one with a flush median less than 1.2-m [4-ft] wide, a raised median is considered unnecessary on short bridges of about 30 m [100 ft] in length but is desirable on bridges of 120 m [400 ft] or more in length. On bridges between 30 m [100 ft] and 120 m [400 ft] in length, local conditions such as traffic volume, speed, sight distance, need for luminaire supports, future improvement, approach cross section,

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