3 Speed Management
3.2.3 I-80 Variable Speed Limit System – Nevada
Impetus for the Activity
The problem that sparked Nevada DOT to consider a technological treatment was localized fog.
Nevada considered a technological treatment for this problem since they already have automatic warning devices for high wind and ice.
The initial application of the system called for an active warning system to address the fog. As the system developed, a field trial of the technology to determine 85th percentile speeds and post them was considered and approved.
Criteria for System Selection (Hardware and Software)
Nevada DOT utilized their existing RWIS to provide the backbone for all of their systems in Northern Nevada.
Description of the System
The system is installed, but is undergoing evaluation before it is activated for traffic. The system was installed 2 years ago, but there have been sensor problems that have precluded activation of the system full time.
The purpose of the system is to set speed limits based on the 85th percentile speed, visibility, and pavement conditions. The system is installed on I-80 next to a coal-fired power generation facility in a canyon with a river. The system consists of:
- Four VSL signs – two eastbound and two westbound;
- Visibility detectors;
- Speed loops;
- RWIS weather station;
- Advance “reduce speed ahead when flashing” flashing signs prior to the installation of the VSL signs – these signs can be used in conjunction with, or independently of, the VSL signs.
For inputs, the system uses 85th percentile speed, visibility (based on stopping sight distance), and pavement conditions (frost, ice, rain, or dry). These inputs are sampled every 15 min. Speed limits are computed using a logic tree, but the signs are limited to increments of 16 km/h [only the tenths digit (in mi/h) can be changed]. The system is remotely controlled with no human intervention, and on clear days, the signs remain blank. The displayed speed is regulatory and thus, enforceable. In the event of low visibility, the posted speed is updated as needed. Otherwise, the output is updated in accordance with the 15-min polling cycle.
System Performance
The reliability of the visibility sensor has limited the operation of this system to date. These systems are complicated and while the technology is sound, the robustness of the field sensors has not met expectations in letting the system function unattended.
System Effectiveness Evaluation
The system had not been activated for use by traffic when the questionnaire was completed. Therefore, no system effectiveness evaluation had been performed.
Perceived “Hot Spots” for Rural ITS Deployment
It was noted on the returned questionnaire that there are definitely “Hot Spots” for rural ITS deployment aimed at improving rural safety, particularly in areas where the detection of hazardous conditions and subsequent warnings can be made. It was also noted that static methods of warning lose their effectiveness over time and that dynamic warnings are much more effective.
Obstacles and Lessons Learned
It was noted that these systems are complicated and they require sound technology, attention to detail, and patience. It was also noted that lawyers should be involved early to thoroughly evaluate any tort liability.