3 Speed Management
The systems falling under Speed Management have been further categorized under:
- Downhill Speed Advisories,
- Variable Speed Limits, and
- Dynamic Curve Speed Warning.
3.1 Downhill Speed Advisories
3.1.1 Dynamic Downhill Truck Speed Warning System – Colorado
Impetus for the Activity
There is a 19 km, 7 percent grade west of the Eisenhower Tunnel on I-70. Commercial vehicle traffic increases annually at 4 to 5 percent and there are similar increases of motor vehicle traffic. The use of the runaway truck ramp for this road segment is the highest in the State, and there is a large number of truck-related crashes.
Colorado State Patrol records show that the majority of truck drivers involved in runaway incidents, or crashes, are either unfamiliar with the road or unfamiliar with mountain driving. Static signs were not effectively communicating the road condition to these drivers. It was determined that a dynamic system that gave a specific message to truck drivers would be more effective.
The expected result of deploying the system was a reduction in the number of runaway trucks or truck-related crashes due to excessive speed.
Criteria for System Selection (Hardware and Software)
International Road Dynamics (IRD) developed the system using an FHWA algorithm, which considered factors of truck weight and grade characteristics, to estimate brake temperature and maximum safe descent speeds.(3 ) The Colorado DOT (CDOT) was familiar with IRD’s work with weigh-in-motion systems.
Description of the System
The initial system was installed 0.4 km west of the Eisenhower Tunnel in 1995 (see Figure 1). After the first year of operation, it was noticed that the pavement sensors were located in an area where trucks often changed lanes, causing a large percentage of missed vehicles. The system was dismantled during a construction project in the summer of 1997.
A new system was designed and installed in 1998.
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 Figure 1: Initial downhill truck speed warning system. |
The system is deployed on a rural section of I-70, west of Denver. It uses inductive loops and Piezo weigh-in-motion sensors in two lanes. Those sensors are tied to a computer inside the Eisenhower Tunnel facility. The computer activates a variable message sign (VMS) that hangs over both lanes of traffic at the west portal of the tunnel. The sensors are located approximately 75 m ahead of the sign. As inputs, the system uses:
- Truck speed,
- Truck weight,
- Axle number, and,
- Axle spacing.
These measures are obtained by the system using the weigh-in-motion sensor, inductive loop detectors, and computer hardware and software, and are used to relay a message for every truck crossing the sensors.
The algorithm within the computer system computes a safe speed based on the truck weight, speed, and axle configuration and displays the recommended speed on a VMS. Each truck receives a vehicle-specific recommended safe speed message. System output is updated continuously and includes:
- Recommended speed,
- Steep grade warning, and
- Low gear recommendation.
The output is advisory.
System Performance
Several factors affect system performance, including:
- Trucks changing lanes at the sensors;
- Quick deceleration at the sensors;
- Pavement wear at the sensors; and,
- Tailgating, or trucks following closely.
The present system has been operating since May 1998 with little maintenance. The personnel of the Eisenhower Tunnel continually monitor the performance and reliability of the system. There has been little need for service and maintenance during the past 2 years.
System Effectiveness Evaluation
The University of Colorado, at Denver’s Transportation Research Center, evaluated the effectiveness of the system and issued a report on December 15, 1999. The key measures of effectiveness examined in this study were driver awareness and compliance.
Since system deployment, truck-related crashes have declined on the steep downhill grade sections, while the volume of truck traffic has increased by an average of 5 percent per year. The system reduces truck descent speeds for most truck weight ranges above 18 100 kg. The mean truck speed with the system turned off was found to be 12.2 km/h greater than with the system on. Truck drivers surveyed responded very positively to the system and its potential to improve safety.
Perceived “Hot Spots” for Rural ITS Deployment
Two specific “Hot Spots” for rural ITS deployment were identified:
- Westbound I-70 at Vail Pass - this road segment has the second highest use of a runaway truck ramp, and
- Westbound U.S. 50 at Monarch Pass - a CDOT traffic safety study recommended this road segment should have a runaway truck ramp because of the number of crashes.
Obstacles and Lessons Learned
A number of obstacles and lessons learned were provided. These include:
- Inductive loops and Piezo sensors are only effective in good pavement. Pavement upgrades and regular maintenance are essential.
- Lightning is a problem on mountain passes. Careful planning for lightning protection is necessary.
- Power and phone hookups can be costly at remote locations.
- The message to the drivers should be personalized. Using “Truck Driver” or “Trucker” in the message will catch their attention and let them know that the message is specifically for them.
- Regularly monitor the weigh-in-motion system to be sure it is calibrated. A phone line to the system computer is handy to monitor the system.
- Local maintenance personnel should be familiar with the system and be able to notify appropriate personnel when the system goes down or is not operating properly.
- The cost of new pavement sensors should be included during any future overlay or pavement upgrade project.