3 Speed Management

3.3.2 Truck Activated Flashers – Missouri

Impetus for the Activity

The problem that sparked the Missouri DOT (MoDOT) to consider a technological treatment was the overturning of several trucks due to excessive speed on a bad curve. MoDOT had several static signs in the area that were not effective in addressing the problem. These signs included an advance curve sign and a reduced speed limit sign (64 km/h).

Traffic studies indicated that the cause of the problem was excessive speeds of tractor-trailer trucks, which caused loads to shift. MoDOT installed a narrow band microwave system that activates wigwag flashers mounted above truck tipping warning signs only when tractor-trailer trucks drive through the area.

The expected result of deploying the system was decreased speed of tractor-trailer trucks approaching the curve.

Criteria for System Selection (Hardware and Software)

MoDOT selected their system based on functionality and cost. They were looking for a system that would target tall vehicles and distinguish their direction of travel.

Description of the System

The system was installed in October 1999 and is currently active. The system is located on a US dual-lane highway in an area with a population of 35,000. The objective of the system is to use an active warning system to cause tractor-trailer trucks to slow down before entering the curve.

The system covers 0.8 km of roadway and includes two signs, two flashers, and one detector. System inputs include direction and height of tall vehicles and are obtained using narrow band microwave detection. Inputs are sampled continuously. The system is activated when tall vehicles enter the bandwidth of the microwave. Detection is not 100 percent accurate, and depends on the shape and composition of the vehicle. The system activates its advisory wigwag flashers when tall vehicles encroach the microwave beam from a single direction. A picture of the system is shown in Figure 6.

Figure 6: Missouri truck-activated flasher system.

System Performance

Composition and shape of the vehicles and sensitivity of setting the detection height are factors that affect system performance. The system is also susceptible to lightning storms.

For the most part, the system has performed satisfactorily, but is not quite as accurate as MoDOT would like. The performance of the system has been informally tested. The measure of performance was how accurately the system targets the tractor-trailer trucks. MoDOT indicated that the system seems to be effective, but sometimes detects shorter vehicles such as motor homes and RVs.

As a result of the variance in the microwave sensor, MoDOT has replaced it with an infrared vehicle detection system.

System Effectiveness Evaluation

No information was provided on whether or not the trucks are slowing down in response to the warning system.

Obstacles and Lessons Learned

Obstacles encountered included the installation of the equipment and the utility costs. One lesson learned is that agencies should be aware that the equipment is not 100 percent accurate.

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