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FHWA Home / Safety / Geometric Design / Publications / Mitigation Strategies For Design Exceptions

Lateral Offset to Obstruction

 

The lateral offset to obstruction is defined as the distance from the edge of traveled way, shoulder, or other designated point to a vertical roadside element.  Examples of these elements are curbs, walls, barriers, bridge piers, sign and signal supports, trees, and utility poles (Figure 25).

Lateral offset can be thought of as an operational offset—vertical roadside elements offset to the extent that they do not affect a driver’s speed or lane position.  Adequate clearance from these elements should be provided for mirrors on trucks and buses and for opening curbside doors where on-street parking is provided.  

The adopted criteria specify a minimum operational offset for all roadway conditions and classifications of 1.5 feet.

Clarification

Lateral offset should not be confused with the clear zone—a clear recovery area, free of rigid obstacles and steep slopes, which allows vehicles that have run off the road to safely recover or come to a stop.  While lateral offset can be thought of as an operational offset, the clear zone serves primarily a substantive safety function.

Figure 25.  Lateral offset to obstruction is an operational offset and is not the same as clear zone.

FIGURE 25  

Lateral offset to obstruction is an operational offset and is not the same as clear zone.

Figure 25 is a photo showing utility poles along the right side of a road that are set back from the travel lane by about a foot.

Lateral offset to obstructions is one of the 13 controlling criteria that require a formal design exception per FHWA Policy.  Clear zone is not.

Although clear zone is not one of the controlling criteria that require a formal design exception, its importance should still be recognized.  The AASHTO Roadside Design Guide provides ranges for clear zone based on speed, traffic, and roadside slopes.  The Guide states that"the values suggest only the approximate center of a range to be considered and not a precise distance to be held as absolute."  Designers are expected to exercise judgment in selecting an appropriate clear zone, taking into account the variables listed above as well as the location (urban vs. rural), the type of construction (new construction/reconstruction/
3R), and the context.  Chapter 10 of the Guide provides guidance on roadside safety in urban and restricted environments and emphasizes the need to look at each location and its particular site characteristics individually.

According to FHWA, a clear zone should be established for projects or project segments based on a thorough review of site conditions, constraints, and safety considerations.  Once a clear zone has been established, decisions to deviate from it for particular roadside obstacles should be identified, justified, and documented.

Summary

Table 21 summarizes the potential adverse impacts to safety and operations of a design exception for lateral offset.

TABLE 21

Lateral Offset to Obstruction:  Potential Adverse Impacts to Safety and Operations

Safety and Operational Issues

Freeway

Expressway

Rural
2-Lane

Urban Arterial

Shying away from obstructions

X

X

X

X

Reduced free-flow speeds

X

X

X

X

Difficulty for parked vehicles

X

Freeway:  high-speed, multi-lane divided highway with interchange access only (rural or urban).
Expressway:  high-speed, multi-lane divided arterial with interchange and at-grade access (rural or urban).
Rural 2-Lane:  high-speed, undivided rural highway (arterial, collector, or local).
Urban Arterial:  urban arterials with speeds 45 mi/h (70 km/h) or less.

Lateral Offset to Obstruction Resources

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Page last modified on April 1, 2019
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