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.

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 48  

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

FIGURE 49  

Dynamic curve warning system.

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

FIGURE 50  

Delineation with large chevrons.

FIGURE 51  

Delineation with post-mounted delineators.

FIGURE 52

Delineation with reflectors on barrier.

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.

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. 

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