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

Horizontal Alignment

 

In terms of the 13 controlling criteria, the term horizontal alignment refers only to the horizontal curvature of the roadway (Figure 14). The adopted design criteria specify a  minimum radius for the selected design speed, which is calculated from the maximum rate of superelevation  (set by policy from a range of options) and the side friction factor (established by policy through research). Superelevation is considered a separate criterion and is discussed below. Horizontal alignment influences another primary controlling criterion, stopping sight distance.

Curve design policy published by AASHTO is based on a series of assumptions of driver behavior and operations. Drivers are assumed to track the curve in a passenger car at design speed. The combination of superelevation, side friction, and radius are established to provide for an acceptable level of comfort for the majority of drivers.  The design model applies to the full range of highway types and conditions.

The radii of curves are one variable that affects the risk of lane-departure crashes on high-speed roadways. Other contributing factors may include the amount of superelevation, the surface friction of the pavement, and the horizontal and vertical alignments preceding the curve. Inadequate superelevation or pavement friction can contribute to vehicles skidding as they maneuver through a curve.  The alignment preceding a curve influences approach speeds. The expected crash frequency increases as the speed differential from the approach tangent to the curve increases.  This may occur if the curve is preceded by a long segment of tangent roadway (versus a continuously curvilinear alignment that encourages lower speeds), if the approach is on a significant downgrade, or if the curve is not visible to the driver on the approach.

At ramps and loops, a lack of deceleration length can contribute to drivers running off the first curve after exiting a freeway.

Horizontal curves can present special safety problems for trucks and other large vehicles.   Because of their higher center of mass, large vehicles are more susceptible to overturning at curves. Research confirms that such overturning can occur at speeds only slightly greater than the design speed of the curve. As discussed in the lane width section, off-tracking of large vehicles onto the adjacent lane or shoulder at horizontal curves can affect the safety of drivers and bicyclists and degrade operations.

The risk of lane-departure crashes at curves is significantly influenced by speed, which is why curves in reduced-speed urban environments generally present fewer safety and operational concerns for the horizontal alignment criterion.

Figure 14.  Horizontal alignment.

FIGURE 14  

Horizontal alignment.

Figure 14 is a photo of complex horizontal alignments in an urban area where two interstate highways converge.

Traffic Operations

Curves influence speed behavior.  Curvilinear roads will have lower speeds, which can negatively affect highway capacity.  However, for some highway types and contexts, lower speeds can be beneficial—for example, reduced-speed urban environments where lower speeds increase safety for pedestrians.  On rural two-lane highways, curves will limit available passing zones and thereby influence capacity.

A curve that is nominally unsafe (has a radius less than the minimum for the selected design speed) may or may not present an unusual operational or safety risk.  Such risk depends on the site conditions. One approach to characterizing this risk for two-lane rural highways is through use of the Design Consistency Module of FHWA’s IHSDM (see Chapter 1). The design consistency module predicts the 85th percentile speed along an alignment as a function of grade, horizontal alignment, roadway width, and direction of travel.

Designers can estimate speeds produced on the approach to a sharp curve to determine the extent of concern over its use or acceptability.  A designer can estimate both the 85th percentile speed through the curve, as well as the change in speeds produced by the alignment of both approaches.  Marginal speed reductions and/or differences between operating and design speed (say, less than 10 mi/hr) may be considered acceptable.

Substantive Safety

The substantive safety performance of a roadway is influenced by the presence and design characteristics of horizontal curvature, including both the length of curve and radius. Other factors contributing to substantive safety of curves include the cross section and the character of the roadside through the curve.

The following AMF can be used to predict how variations in horizontal alignment will affect the expected safety performance of rural two-lane highways:

AMF = (1.55Lc + 80.2/R – 0.012S)
                1.55Lc

Where,

Lc = length of horizontal curve (mi)

R = radius of curvature (ft)

S = 1 if spiral transition curve is present
   = 0 if spiral transition curve is not present

The difference in substantive safety between two designs can be estimated by comparing the result of exercising this function for the two cases and comparing the results. Note that at a given location the curve’s central angle will be fixed, and hence a milder curve than the alternative will be longer.

Note that the effect on total safety risk will vary with traffic volume as well. Designers may accept a design exception for curvature on a roadway with a design volume of 750 vehicles per day (vpd), but reach a different conclusion for a road with a design volume of 8,000 vpd.

Summary

Table 11 summarizes the potential adverse impacts to safety and operations of a design exception for horizontal alignment.

 

TABLE 11

Horizontal Alignment:  Potential Adverse Impacts to Safety and Operations

Safety & Operational Issues

Freeway

Expressway

Rural
Two-Lane

Urban Arterial

Run-off-road crashes

X

X

X

 

Cross-median crashes

X

X

   

Cross-centerline crashes

   

X

X

Large vehicle rollover crashes

X

X

X

 

Large vehicles off-tracking into adjacent lane or shoulder

X

X

X

X

Skidding

X

X

X

X

Rear-end crashes if operations deteriorate (abrupt speed reduction)

X

X

X

 

Reduced free-flow speeds

X

X

X

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.

Horizontal Alignment Resources

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Page last modified on October 15, 2014.
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