Overview
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.
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.
Curve Advisory Speed Software
Download the CAS Software [XLS, 634 KB]
You may need the Excel Viewer to view this XLS.
This part of the chapter provides an overview of the Curve Advisory Speed (CAS) software. This spreadsheet was developed to automate the procedures and guidelines described in this handbook. The background for the development of the equations in this spreadsheet is documented in an earlier research report by Bonneson et al. (5). The current Curve Advisory Speed (CAS) software accommodates several methods for establishing advisory speeds and follows the curve signing criterion according to the MUTCD 2009 edition (3).
The "Analysis" tab worksheet contains the curve advisory speed calculations. This worksheet is shown in Figure 4. Six (6) columns are provided in the worksheet. One column is used for each curve being evaluated.
Figure 4 – Curve Advisory Speed (CAS) Software Analysis Worksheet
The spreadsheet can be used with six types of input data. One method is based on data obtained from a survey of the curve using the Compass Method. This method is described in Chapter 3. The drop-down list located in cell F5 is used to specify this method by selecting "Compass," as shown in Figure 4. The data from the Compass Method are entered in the cells that have a light blue shaded background in the rows 9 through 21 and are designated "input data" cells. If the 85th percentile tangent speed is not known, then this cell should be left blank, and row 25 and the estimate in the row 22 (or the speed limit in the row 21, whichever is larger) will be used as the 85th percentile speed. The cell in the row 25 (orange shaded) is optional to input the 85th percentile speed of free-flowing passenger cars on the tangent prior to the curve. The information in the rows 31 to 34 (light blue shaded) are required to account for special roadway configuration.
The second and third methods of data input are based on describing the curve deflection angle, superelevation rate, and radius. These data can be obtained from the GPS Method or the Design Method, as described in Chapter 3. The use of either of these methods is specified with the drop-down list located in cell F5 by selecting "GPS" or "Design." The data from the GPS Method or the Design Method are entered in the light blue shaded cells in the row 21 and 26 through 29. If the 85th percentile tangent speed is not known, then this cell should be left blank, and the estimate in the row 22 (or the speed limit in the row 21, whichever is larger) will be used as the 85th percentile speed. The cell in the row 25 (orange shaded) is optional to input the 85th percentile speed of free-flowing passenger cars on the tangent prior to the curve. The information in the rows 31 to 34 (light blue shaded) are required to account for special roadway configuration.
The fourth, fifth and sixth methods of data input are based on other methods, such as the Direct Method, the Ball-Bank Indicator Method, and the Accelerometer Method, as described in Chapter 3. The use of any of these methods is specified with the drop-down list located in cell F5 by selecting "Direct," "Ball-Bank Indicator," or "Accelerometer." The advisory speed established by the other method is entered directly in the light blue shaded cells in the row 48. The cell in the row 21 (regulatory speed limit) is required to apply MUTCD 2009 edition (3) signing criteria. The information in the rows 31 to 34 (light blue shaded) are required to account for special roadway configuration. It is optional to input the 85th percentile speed of free-flowing passenger cars on the tangent prior to the curve in the cell in row the 25 (orange shaded). If the 85th percentile speed is not known, then this cell should be left blank, and the estimate in the row 22 (or the speed limit in the row 21, whichever is larger) will be used as the 85th percentile speed. The information, total curve deflection angle, in the cell in the row 26 (light blue shaded) is used for curve warning sign determination process.
The cells in the rows 36 to 46, which do not have background shading, contain equations for the first three input methods. The basis for each equation is documented in Bonneson et al. (5). These equations document the analysis of advisory speed for each of the six curves. The purple shaded cell in the row 49 is the advisory speed established by the applied method.
The purple shaded cells in the rows 54 to 73 of the spreadsheet document the traffic control device guidance. The criterion described in Chapter 4 of this manual and the MUTCD 2009 edition (3) are used to calculate the information that is summarized in this section of the spreadsheet. If cells are blank in this section of the spreadsheet then the traffic control device for that column is not required for that curve.
The cells in the rows 94 to 107 of the spreadsheet contain many parameters that control the computations. The default model used for calculating the advisory speed is based on the average truck speed (5). The corresponding formulation is as follows:
where,
Rp = travel path radius, ft;
Vc,85 = 85th percentile curve speed, mph;
Vt,85 = 85th percentile tangent speed, mph;
Vc,a = average curve speed, mph;
Vt,a = average tangent speed, mph;
e = superelevation rate, percent; and
Itk = indicator variable for trucks (= 1.0 if model is used to predict truck speed; 0.0 otherwise)
Note that the default model used in the Curve Advisory Speed (CAS) software assumes to use the estimated average truck speed to establish the advisory speed for a particular curve. This is a conservative approach proposed by TTI to ensure safety. However, there is no consensus or agreement among various transportation practitioners on whether to use passenger car vs. truck and average speeds vs. 85th percentile speeds as the criteria to establish advisory speeds. Therefore, it is suggested, before modifying these parameters in the spreadsheet, to carefully read through the report by Bonneson et al. (5) and think about if the default assumption matches your engineering experience and judgment.
If other criteria are chosen to establish the advisory speed, the parameters can be modified. For example, if the 85th percentile speed is preferred to be used as the advisory speed, the formulation shall be changed to the following equation.
The formulation is changed through the modification in the cell in the row 103.
If the passenger car speed is preferred to be used to establish the advisory speed, due to low volume of trucks or low truck crash rate, the value in the cell in the row 97 shall be changed to 1.0 and Itk in the formulation shall be set to 0.0.
More information:
http://safety.fhwa.dot.gov/speedmgt/ref_mats/fhwasa1122/
