3 Speed Management
3.2.2 Snoqualmie Pass – Travel Aid – Washington
Impetus for the Activity
The variable speed limit system (VSL) was proposed to reduce winter weather-related crashes. Initial analysis showed a three-fold increase in crashes during the winter months when Snoqualmie Pass frequently experiences snow and sub-freezing temperatures. Crash rates during snow events were more than four times the annual average. A large portion of these crashes involved run-off-the-road, rear end, and sideswipes, with the apparent causes being driving too fast for conditions and large differences in speeds.
Washington State DOT (WSDOT) South Central Region Operations was seeking a way to better communicate conditions and problems (such as roadway closures) to the traveling public. Discussions with WSDOT ITS program arrived at a solution that addressed that need, as well as a way to address the crash problems. No “static” solution would offer the flexibility to provide a wide range of information and allow for dynamic speed limits to be set without considerable work force to change fixed signs.
Criteria for System Selection (Hardware and Software)
The Travel Aid proposal to FHWA was made jointly by WSDOT and PB Farradyne as a team. As such, the system’s design, component integration, and MIST central system development was done by PB Farradyne as part of the proposing team. Roadside elements were designed by WSDOT and were installed using a traditional construction contract. Radio communications were set up and tested by WSDOT radio personnel.
Most system components were specified by the consultant and were specified along with the “or equal” clause. These components included the OAC (which acts as a communications arbitrator at roadside installations), radar detectors, and radio communications equipment. WSDOT purchased the VMS signs through a separate request for proposal process and furnished them to the construction contractor for installation.
Description of the System
The system is currently installed and active. VSL operations first started in December 1997. VSL is typically operated during the winter months from early November to early April. The VMS signs are used to support construction and maintenance activities in the summer.
The objective of the system is to improve safety and to increase the availability of road condition and weather information to motorists crossing Snoqualmie Pass
The system is deployed in a rural and mountainous environment. I-90 Snoqualmie Pass crosses the Cascade mountain range and is the main east-west route across Washington State. The pass summits at 921 m and includes an 8-km segment with a 5 percent grade as it approaches the summit. West of the summit, three lanes are provided in each direction, and two lanes in each direction east of the summit area. Thirty percent of traffic includes trucks.
Much of the moisture from Seattle’s famous marine climate precipitates along the Cascade Range’s western slopes. A typical year will have over 225cm of rain, and in the winter it often results in considerable snow accumulation.
The system includes 13 Light Emitting Diode (LED) VMSs over 64 km, but only 27 km (mp 45 to mp 62) are operated as VSL during the winter months. Six weather stations provide environmental conditions and there are sensors in the pavement to determine pavement conditions. The system has 22 radar speed detectors
For inputs, the system uses whether or not traction tires are advised, traction tires are required, or whether or not chains are required, and other elements including visibility and severe weather. Road Weather Information System (RWIS) stations are poled every 30 min. Radar speed detectors provide a smoothed output every 5 min. Newer radar products that aggregate data into speed and classification bins may have provided more useful information for VSL algorithms and analysis.
 Figure 2: A VMS on the Snoqualmie Pass VSL system. |
The decision to reduce the speed limit is based on feedback from multiple weather stations, snowplow operators, and State Patrol. The speed limit throughout the Pass is posted at 105 km/h due to roadway geometry. When roadway conditions are poor, speed limits are reduced in 16-km/h increments, primarily depending on whether traction tires are advised (89 km/h), traction tires are required (72 km/h), or whether chains are required (56 km/h). WSDOT has also developed a matrix of speeds based on other elements including visibility and severe weather. The speed limit is recommended by computer and confirmed by an operator. The system output consists of posted speed limit and road condition messages, which are updated as conditions change. The speed limits and condition messages are made available through VMSs, and the speed limits are enforced. The goal is to reduce crashes caused by driving too fast for conditions and by reducing the range of vehicle speeds to reduce conflicts. A picture of the system is shown in Figure 2. |
System Performance
In terms of factors affecting system performance, radar detectors have failed at an alarming rate, as the housings are filling with water and corroding the electronics inside. WSDOT is in the process of identifying other manufacturers that can provide a compatible output. All other portions of the system are functioning satisfactorily. The VMSs have not had a failure in over 3 years.
The performance and reliability of the system has not been formally tested, but maintenance records will be examined, and a report will be generated on system performance.
System Effectiveness Evaluation
A comprehensive evaluation of the project will be performed by the Washington State Transportation Center. The emphasis of this study is to evaluate crash reductions and compliance to the VSL. This report has been delayed due to the unavailability of crash data at this time. WSDOT has observed motorists slowing down when the VSL system is in use.
Perceived “Hot Spots” for Rural ITS Deployment
Expert opinions suggest looking for locations with high crash histories or chronic problems that might be addressed by ITS. In addition, while it would be good to apply ITS everywhere, there remains a need to address the worst locations first (where the highest payoffs will be achieved).
Obstacles and Lessons Learned
A paper entitled, “ITS Field Operational Test Contracting: Avoiding Surprises,” was produced by Larry Senn and Eilert Bjorge.5 This paper offers insights into obstacles encountered and lessons learned on the Travel Aid Field Operational Test. These lessons learned are mainly aimed at agencies putting together a proposal for an FHWA Field Operational Test. A few are included in the following list:
- Make sure funding is available for all aspects of the Field Operational Test;
- State funds must be available for operations and maintenance, or they should be included in the Field Operational Test Proposal;
- When making an estimate, keep in mind that any project that involves a lot of research and development can experience cost escalation and scope creep;
- Many problems may be headed off by simply going over contracting procedures and requirements during the proposal development;
- Get started early on the contract negotiations in earnest once the FHWA has named your project as one it has selected; and
- Have your cost proposal checked out by the State DOT’s audit section for acceptable fee and overhead rates.