SafeCurves: Interventions - Primer

Horizontal curves

Curve advisory speeds and warning signs

Road agencies can apply a range of different countermeasures to horizontal curves to address safety problems. The purpose of this SafeCurves Primer is to answer questions about curve advisory speeds and warning signs and their importance in horizontal curve safety.  

What is the problem?

The USA has a big road safety problem, with more than 42,000 people killed on US roads each year (National Center for Statistics and Analysis, 2023). At 12.61 deaths per 100,000 people, the US is significantly underperforming when compared to other developed countries (Institute for Health Metrics and Evaluation, 2019).  

Managing the safety of road users traversing horizontal curves is a major issue for road agencies globally. In the United States, over 25% of all fatal crashes occur on horizontal curves. The average crash rate on horizontal curves is approximately three times higher compared to straight sections of road (FHWA, 2023). 

What causes crashes on horizontal curves?

Research suggests that the primary causes of crashes on horizontal curves include:

• Complex geometry;

• Excessive speed;

• Drivers’ unawareness and misperception of the curvature of the approaching curve; and

• Environmental conditions, such as visibility or weather.
  

Horizontal curve treatments

Horizontal alignment signs and plaques

Given that drivers’ unawareness is a key cause of crashes at horizontal curves, conveying information to drivers about upcoming curves is critical. The primary objective of warning signs is that they give drivers sufficient time to react, so they can either alter the path and/or speed of their vehicle to negotiate the curve safely.

Advance warning signs

An advance warning sign alerts a driver to the change in alignment and provides information on whether the alignment turns to the left or right downstream of the sign. There are many types of advance warning signs which road agencies can apply to horizontal curves (Figure 1).  

Figure 1: Horizontal alignment signs and plaques. Source: 11th Edition MUTCD

For a single curve section, the following signs can be used:

• Turn (W1-1)

• Curve (W1-2)

• Hairpin Curve (W1-11)

• 270-degree Loop (W1-15)

For sections with more than one curve in a sequence of curves, the following signs can be used:

• Reverse Turn (W1-3)

• Reverse Curve (W1-4)

• Winding Road (W1-5)

Advisory Speed Plaque

An Advisory Speed plaque (W13-1P and W13-1aP) informs the driver that they may need to adjust their speed and the speed that is most appropriate to the curve.  

Chevrons

Chevrons (W1-8) are used to delineate a curve and convey curve sharpness to drivers. Chevrons have been shown to reduce nighttime crashes on curves by 25% (Albin, 2016).

Challenges with horizontal alignment warning signs

Although signage is critical to improve road safety at horizontal curves, research shows that overuse or inconsistent use of curve warning signs and Advisory Speed plaques makes it difficult for drivers to develop expectancies, and consequently promotes disrespect for the sign and mistrust of its message (Milstead, 2011; National Academies of Sciences, Engineering, and Medicine, 2004).  

For example, Leaming (2014) notes: 
“Region investigators, county engineers, other state DOTs, and researchers have all reported existing signing is so inconsistent that they are an unreliable measure of actual signing needs.” 

To mitigate these challenges, the FHWA have developed procedures to:

• Identify when a curve warning sign and advisory speed are needed, through the Manual for Uniform Traffic Control Devices for Streets and Highways (MUTCD); and

• Select an advisory speed that is consistent with driver expectation, through the Curve Advisory Software (CAS) (Milstead, 2011).

MUTCD: Sign and plaque standards

Providing uniform, consistent traffic control devices is a core purpose of the new MUTCD 11th Edition (FHWA 2023).  
“Uniformity of the meaning of traffic control devices is vital to their effectiveness. Uniformity means treating similar situations in a similar way. Uniformity of devices simplifies the task of the road user because it aids in recognition and understanding, thereby reducing perception/reaction time.” 

The MUTCD sets out specific recommendations and requirements for the installation of curve warning and advisory speed signs. The purpose of the MUTCD is to ensure uniform and consistent display of signage and advisory speeds on curves of similar geometry and character.
The MUTCD requires the use of a warning sign be based on engineering study or engineering judgement. The FHWA (Albin, 2016) recommends practitioners consider the following key factors when determining the application of curve warning signs:

• The difference between the posted speed limit and recommended advisory speed;

• Geometric features of the curve;

• Sight distance to and around the curve;

• A sudden change in alignment after many miles of consistently straight roadway;

• Traffic volume; and

• Crash data.

Setting curve advisory speeds

The Curve Advisory Software (CAS) automates the procedures and guidelines when selecting an appropriate plaque advisory speed. The FHWA recognise that a uniformly applied advisory speed will be more likely to command the respect of drivers and achieve the desired safety benefits.  

Historically, road agencies have identified curve advisory speeds using several different methods:  

• Direct Method,

• TTI Curve Speed Model – Compass Method,

• TTI Curve Speed Model – Global Positioning System (GPS) Method,

• TTI Curve Speed Model – Design Method,

• Ball-Bank Indicator Method, and

• Accelerometer Method.

Ball-bank indicator testing is the most common method to determine advisory speeds on curves. This involves driving selected corridors and curves using an inclinometer mounted in a vehicle. Readings are taken at different speeds along a curve to determine safe travelling speed. However, reading the ball-bank indicator to determine the maximum degree of lean can be subjective (Milstead, 2011). This is consistent with research which shows that variability in ball-banking readings can result in advisory speeds which vary by 5-10 mph among curves of similar geometry and condition (Bonneson, 2007). Moreover, driving curves in-field is time consuming, expensive and is not conducive to network-wide analysis. Further, traditional ball banking methods have limited study documentation and uniformity of data. Measurements can be impacted by situational data elements such as the weather, traffic operations or congestion.  

Although the disbenefits of ball-banking are well understood, ball-banking remains a stable tool for engineering practitioners because of its ease of accessibility, inexpensiveness, and simplicity of use.  

Future practice

Abley’s SafeCurves: Interventions – Speed advisory and warning signs is a data product that assists road safety professionals with a consistent and targeted approach to determine advisory speeds and signs for all curves. It follows the Federal Highway Administration (FHWA) procedures for setting advisory speeds on curves (Milstead, 2011), and the 11th Edition of the Manual for Uniform Traffic Control Devices for Streets and Highway (MUTCD) standards for horizontal alignment sign selection and placement. The output is a geospatial dataset containing speed advisory information and warning sign placement for all relevant curves across a network.  

SafeCurves: Interventions – Speed advisory and warning signs enable road agencies to deliver consistent, uniform signage to drivers and, in doing so, build trust in the message. This will impact driver behaviour and increase the efficacy of curve warning signs and advisory speeds as a horizontal curve treatment. The expected outcome is reduced loss of control crashes and an increase in road safety.   

When SafeCurves: Interventions – Speed advisory and warning signs is coupled with SafeCurves: Prioritize the road safety practitioner, traffic engineer, or maintenance and operations teams are further benefited through a work program that focus on compliance as well as directing priority towards road safety needs.   

Revision V1.4 
Issue Date ​​​30 Oct 2024​ 
SafeCurves version 2024.2 

References

Albin, R., Brinkly, V., Cheung, J., Julian, F., Satterfield, C., Stein, W., Donnell, E., McGee, H., Holzem, A., Albee, M., Wood, J., Hanscom, F. (2016) Low-Cost Treatments for Horizontal Curve Safety. Federal Highway Administration. Washington, DC. FHWA-SA-15-084 

Bonneson, J, M. Pratt, J. Miles, and P. Carlson. (2007) Development of Guidelines for Establishing Effective Curve Advisory Speeds. Texas Department of Transportation, Texas. FHWA/TX-07/0-5439 1 

Dixon, K., Rohani, J. (2008) Methodologies for estimating advisory curve speeds on Oregon highways. Oregon State University, Oregon. FHWA-OR-RD-08-04 

Federal Highway Administration (2023) Manual on Uniform Traffic Control Devices for Streets and Highways 11th Edition. Available at: https://mutcd.fhwa.dot.gov/pdfs/11th_Edition/mutcd11thedition.pdf Accessed 14 March 2024.  

Federal Highway Administration (2023) Horizontal Curve Safety. Available at: https://highways.dot.gov/safety/rwd/keep-vehicles-road/horizontal-curve-safety Accessed 21 December 2023.  

Institute for Health Metrics and Evaluation, Global Burden of Disease (2019) – processed by Our World in Data. Available at: https://ourworldindata.org/grapher/death-rates-road-incidents Accessed 7 February 2024.   

Leaming, E. S. (2014). Curve Evaluations using Rieker’s CARS. Shakedown and development of methodology. Oregon Department of Transportation. 

Milstead, R., Qin, X., Katz, B., Bonneson, J., Pratt, M., Miles, J., Carlson, P. (2011) Procedures for Setting Advisory Speeds on Curves. Federal Highway Administration. Washington, DC. FHWA-SA-11-22 

National Academies of Sciences, Engineering, and Medicine (2004) A Guide for Reducing Collisions on Horizontal Curves. The National Academies Press. Washington, DC.  

National Center for Statistics and Analysis (2023) Early estimate of motor vehicle traffic fatalities in 2022 (Crash Stats Brief Statistical Summary. Report No. DOT HS 813 428). National Highway Traffic Safety Administration. Washington, DC.