U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
202-366-4000


Skip to content
FacebookYouTubeTwitterFlickrLinkedIn

Safety

eSubscribe
eSubscribe Envelope

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

5. Horizontal Alignment and 6. Superelevation

Horizontal alignment and superelevation strategies have been combined in this discussion because they are normally evaluated in combination.  The two criteria are also interrelated in terms of their effects on safety and operations.  

Signing and Pavement Marking Messages

Target areas:  Any highway, particularly high-speed highways, at the approach to sharp or unexpected horizontal curves.

Strategy:  Advance warning with signing and pavement markings.

Signs can be used to warn drivers in advance of sharp horizontal curves and where there is non-standard superelevation (FIgures 47 and 48).  The most commonly used are the curve warning sign (for advisory speeds of 30 mi/h or greater) and the turn warning sign (for advisory speeds less than 30 mi/h).    Advisory speed plaques mounted below the warning sign are often used.  In some situations, flashers installed in conjunction with the sign may further increase driver awareness.  The MUTCD provides guidance on the size of warning signs for various highway types but notes that larger signs may be used when appropriate.  Larger warning signs should be considered for design exception locations.

Another consideration, besides the radius of the curve and the rate of superelevation, is the roadway alignment leading up to the curve.  For example, a curve on a highway with a predominantly curvilinear alignment is more expected by the driver.  Conversely, a sharp curve along a highway with a predominantly straight alignment or at the end of a long tangent is more likely to surprise a driver.  Advance warning is especially important in these situations.

Curve warning messages painted on the pavement are another method for providing advance warning of horizontal curves.  One example is the painted message SLOW, along with a painted turn arrow.

Dynamic Message Signs

Target areas:  Curves with a history of safety problems.  A common application is to mitigate truck rollover crashes on sharp curves at interchange ramps and loops.

Strategy:  Dynamic message signs.

At some curves, signs that provide dynamic messages to drivers may be an effective countermeasure (Figure 49).  Changeable, real-time information can be communicated to the driver, such as the current recommended speed and the driver&rsquo s current operating speed.

Figure 47.  Turn warning sign with flashing beacon.

FIGURE 47  

Turn warning sign with flashing beacon.

Figure 47 is a photo showing a turn warning sign with three elements mounted vertically on a post along a curve to the right in the road.  The bottom part consists of a square sign with a black border and the legend 10 MPH on a yellow background.  The middle part consists of a diamond-shaped sign with a vertical black arrow, on a yellow background, bent at a 90-degree angle pointing to the left. The top element consists of a flashing yellow beacon.

Figure 48.   Curve warning sign.  Note how vertical alignment can affect visibility of the curve.

FIGURE 48  

Curve warning sign.  Note how vertical alignment can affect visibility of the curve.

Figure 48 is a photo showing a head-on view of a road, rising away from the viewer, with a curve warning sign with two elements mounted vertically on a post to the right of the road.  The bottom part of the warning sign consists of a square sign with a border and the legend 40 MPH in black on a yellow background.  The top part consists of a diamond-shaped sign with a vertical black arrow, on a yellow background, curving up and to the left. The vertical alignment of the road hides the horizontal curve from view.

Figure 49. Dynamic curve warning system.

Figure 49. Dynamic curve warning system.

FIGURE 49  

Dynamic curve warning system.

Figure 49. Dynamic curve warning system.

Figure 49 consists of three photos. The first photo shows a square-shaped sign with an arrow curving to the left and down over a symbol of a truck tipped to the right at a 45-degree angle. Below this sign is mounted a changeable message sign displaying the message TRUCKS REDUCE SPEED.  The second photo shows another changeable message sign displaying the message YOUR SPEED 63 MPH  and the third photo shows the same changeable message sign displaying the message 60 MPH CURVES AHEAD.

Delineation

Target areas:  Any sharp or unexpected horizontal curve.

Strategy:  Delineation

In addition to advance warning, delineation is a common mitigation strategy for horizontal curves.  There are several ways to effectively delineate horizontal curves:

  • Chevrons (FIGURE 50).  The MUTCD provides guidance on chevron size for various highway types but notes that larger signs may be used when appropriate.  Larger chevrons should be considered for design exception locations.

  • Post-mounted delineators (FIGURE 51).

  • Reflectors on barrier.  If barrier is used along the horizontal curve, low-cost delineation can be provided with reflectors installed along the barrier (FIGURE 52).  

Figure 50.  Delineation with large chevrons.

FIGURE 50  

Delineation with large chevrons.

Figure 50 is a photo showing traffic on a road with a series of signs mounted atop a concrete barrier on the left side of the road along a horizontal curve.  Each sign is square in shape with a yellow background and a black chevron pointing to the right to indicate the direction of the curve.  The chevrons are larger than the standard size.
 

Figure 51.  Delineation with post-mounted delineators.

FIGURE 51  

Delineation with post-mounted delineators.

Figure 51 is a photo showing a post&ndash mounted delineator&mdash a circular reflective disk mounted on a post along the side of the road.

Figure 52.  Delineation with reflectors on barrier.

FIGURE 52

Delineation with reflectors on barrier.

Figure 52 is a nighttime photo showing illuminated reflectors installed along w-beam guardrail.

Widen the Roadway

Target areas:  Curves on highways with large truck volumes, cross-centerline crashes, or run-off-road crashes.

Strategy:  Widen the roadway.

Widening the travel lanes at horizontal curves can mitigate off-tracking of trucks and other large vehicles into adjacent lanes.  Additional lane width will make it easier for all drivers to maneuver through the curve without leaving the travel lane.  If cross-centerline crashes are a problem at a curve, a narrow median, preferably with centerline rumble strips, can provide some separation between the directions of traffic.  If run-off-road crashes are more prevalent, widening the shoulder will help a driver that has left the travel lanes safely recover.  Lane widening can also be beneficial on ramps and loops, particularly where there is a history of run-off-road crashes.  The AASHTO Policy on Geometric Design of Highways and Streets provides design guidance on lane widening through curves.

Skid-Resistant Pavement

Target areas:  Any horizontal curve.

Strategy: Grooved, textured, or open-graded pavements to improve surface friction and skid resistance.

Another strategy aimed at keeping drivers on the roadway is to provide pavement treatments to improve surface friction and skid resistance such as grooving of PCC pavement and open-graded friction courses for HMA pavement.  Pavement grooving and other textures (Figures 62 and 63) can be placed at the time pavement is constructed or they can be milled into existing pavement.  See the Cross Slope section for more information. 

Other Horizontal Curve Strategies

Because horizontal curves are a contributing factor to lane departure crashes, many of the strategies for preventing or reducing the severity of these crashes are applicable.  See the Lane and Shoulder Width discussion earlier in this chapter for additional information on the following strategies:

  • Enhanced pavement markings

  • Lighting

  • Shoulder, centerline, and painted edgeline rumble strips

  • Paved or partially paved shoulders

  • Safety edge

  • Clear recovery area, traversable slopes, breakaway safety hardware, and barrier where appropriate

Target areas:  Any horizontal curve.

Strategy:  Preventing or reducing the severity of lane departure crashes.


Back to the Table of Contents
Page last modified on April 1, 2019
Safe Roads for a Safer Future - Investment in roadway safety saves lives
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000