Icy Curve Warning Systems in California reduced the number of annual crashes by 18 percent, and the system was estimated to provide safety benefits of $1.7 million dollars per winter season.

Researchers found that deployment of ICWSs reduced crashes and improved safety.

Date Posted

Best Practices for Road Weather Management, Version 3

Summary Information

In June 2012 U.S. DOT finalized Version 3.0 of the Best Practices for Road Weather Management report. This report contains 27 case studies of systems in 22 states that improve roadway operations under inclement weather conditions. Each case study has six sections including a general description of the system, system components, operational procedures, resulting transportation outcomes, implementation issues, as well as contact information and references.

The previous report, Best Practices for Road Weather Management Version 2.0 presented 30 case studies from municipal and state transportation agencies. At this point, those solutions are either mainstreamed or have been surpassed by even better solutions. The Version 3.0 report captures the state-of-the-art, presenting 27 all-new practices that build upon these agencies’ previous successes.

Fredonyer Pass, located in northeastern California, is a five-mile segment of State Highway 36 in Lassen County that has a history as a high-collision location, including multiple fatal crashes involving local residents. The vast majority of these crashes occurred when the pavement was icy, despite static signage that the California Department of Transportation (Caltrans) had installed to increase motorist awareness.

To address this, Caltrans deployed an Icy Curve Warning System (ICWS) consisting of pavement sensors to detect icy conditions, in combination with dynamically activated signage to provide motorists with real-time warning when icy conditions are either imminent or present. The intention of the system was to use real-time messaging to increase motorist vigilance and reduce the number of crashes occurring during icy pavement conditions. It alerts motorists of icy conditions, eliciting a decrease in vehicle speeds during such conditions. Consequently, lower vehicle speeds are expected to translate to reduced crashes along the length of the curves which have presented safety challenges in the past.

System Components and Operations: Each system consists of a Road Weather Information System (RWIS) with roadway sensors that continuously monitor the road surface condition and identify when icy or packed snow conditions are present, and two Extinguishable Message Signs (EMS) with flashing beacons, that provide dynamic warnings to motorists when icy or packed snow conditions are present. Specialized scripts are executed in the RWIS Remote Processing Unit (RPU) that evaluate the status of the roadway sensors and determine when the ice warning should be active. A signal from the RPU then turns on the warning signs. At the summit system, a Closed Circuit Television (CCTV) camera is also present to allow the TMC in Redding to monitor conditions at the pass.

One RWIS is placed in the heart of each curve at a location determined by to experience icing conditions most frequently. One EMS was placed on the approaches to each curve at a location to provide adequate braking distance for vehicles headed into an icy curve.


In order to determine the safety effects of the ICWS, an observational before-after study using the Empirical Bayes technique was employed. This evaluation determined the effect of ICWS on crash frequencies.

  • The results found that the deployment of the ICWS reduced the number of annual crashes by 18 percent. As no other changes occurred along the study segment (additional safety improvements, geometric changes, etc.), it is reasonable to attribute this observed safety improvement to the ICWS.
  • Additionally, a crash rate method was used to investigate the effect of the ICWS on crash severities, with a focus on ice-related accidents. The results indicated that the ICWS has reduced crash severities.
  • As an outcome of reduced crash severities, the system was estimated to provide safety benefits of $1.7 million dollars per winter season during the after deployment study period (2008-2009, on account of time lag in crash data availability). Given that 1.5 years of after-period data was available for analysis, it would be advisable to revisit the safety performance of the Fredonyer ICWS at some point in the future when more years of crash data are available. Overall however, the initial safety evaluation results indicate that the system is having a positive impact on reducing crashes.
From the perspective of winter maintenance personnel, the ICWS is an improvement over the typical static metal signage. Observations made over time have indicated that as the winter progresses, the system works better. The use of additional pavement sensors for detection of conditions in multiple lanes could improve system accuracy and reliability. The data produced by the ICWS is not presently employed by maintenance forces for any activity, although the CCTV camera associated with the system’s RWIS at the summit is used frequently to obtain visual information on present conditions.

The full report, finalized in June 2012, assesses many strategies for Road Weather Management. These strategies improve safety, efficiency, and mobility. These findings along with the benefits and costs provide a valuable resource to those considering the implementation of Road Weather Management systems.
Goal Areas
Deployment Locations