Horizontal curves are a necessary component of the highway alignment; however, they tend to be associated with a disproportionate number of severe crashes. Recently, in the United States, about 33,000 fatalities occur nationwide each year, and about 25 percent of these fatalities occur on horizontal curves (2).
Curve warning signs are intended to improve curve safety by alerting the driver to an upcoming change in geometry that may not be apparent or expected. One or more of the curve warning signs identified in the Manual on Uniform Traffic Control Devices (MUTCD 2009 edition) (3) are typically used to notify drivers. Drivers may also be notified of the need to reduce their speed through the use of an Advisory Speed plaque.
Several research projects conducted in the last 20 years have consistently shown that drivers are not responding to curve warning signs and are not complying with the Advisory Speed plaque. Evidence of this non-responsiveness is supported by the aforementioned curve crash statistics. Chowdhury et al. (4) suggest that current practice in the U.S. for setting advisory speeds is contributing to this lack of compliance and the poor safety record. They advocate the need for a procedure that can be used to: (a) identify when a curve warning sign and advisory speed are needed, and (b) select an advisory speed that is consistent with driver expectation. They also recommend the uniform use of this procedure on a nationwide basis, such that driver respect for curve warning signs is restored and curve safety records are improved.
1.2 Purpose and Scope
The procedures described in this handbook are intended to improve consistency in curve signing and, subsequently, driver compliance with the advisory speed. The handbook describes guidelines for determining when an advisory speed is needed, criteria for identifying the appropriate advisory speed, engineering study methods for determining the advisory speed, and guidelines for selecting other curve related traffic control devices.
The handbook is to be used by traffic engineers and technicians who are responsible for evaluating and maintaining horizontal curve signing and delineation devices.
The curve advisory speed and other curve related traffic control devices should be checked periodically to ensure that they are appropriate for the prevailing conditions. Changes in the regulatory speed limit, curve geometry, or crash history may require an engineering study to reevaluate the appropriateness of the existing signs and the possible need for additional signs.