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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Public Roads - Spring 1994

An Automatic Warning System to Prevent Truck Rollover on Curved Ramps

by Hugh W. McGee and Rodney R. Strickland


Truck accidents on urban freeways occur more frequently at interchanges -- particularly on curved exit ramps -- than at any other location. In fact, trucks overturning on exit ramps at interstate interchanges account for five out of every 100 fatal truck accidents. (1) Truck rollover accidents can be very costly, especially in urban areas, because these accidents usually result in fatalities and injuries, vehicle and roadway damage, and traffic delays. Losses are even greater when trucks carrying combustible or hazardous cargo are involved.

One way to prevent -- or at least reduce -- truck rollover accidents on curved exit ramps would be to install an automatic warning system on these ramps to help truck drivers take preventive action. The system warns drivers when the truck, based on its load conditions and speed, would roll over if its speed were not reduced.

"Feasibility of an Automatic Truck Warning System," a Federal Highway Administration (FHWA) study, looked at the details of creating and implementing such an automated warning system. The study team developed system requirements, prepared design plans and specifications, identified available system hardware and software, and installed a prototype truck warning system at selected ramps on the Capital Beltway in Maryland and Virginia. The team then assessed the costs and benefits of the system. These research activities and assessments are summarized in this article.


Example of a static warning sign and an activated warning sign used together.


The Rollover Process

As a truck travels through a curved ramp, its speed and the ramp's curvature and superelevation cause a level of lateral acceleration on the truck. For each truck and loading condition, there is a maximum value of lateral acceleration beyond which it will roll over. This level of acceleration is called the rollover threshold (RT). The University of Michigan Transportation Research Institute (UMTRI) has developed rollover threshold values for various trucks and loading conditions using static and dynamic tests. (23) These values are shown in table 1. UMTRI also has defined the maximum lateral acceleration a truck with a given rollover threshold can sustain:

aymax= (RT - SM)/  1.15

where SM is a safety margin value and 1.15 is a factor accounting for additional lateral acceleration due to steering fluctuations during the turn. (3)



Truck Rollover Occurrence on Capital Beltway Ramps

The present study involved 41 interchanges on the Capital Beltway in Virginia and Maryland. Ramp locations atwhich truck rollovers occurred were identified from accident data, including police reports, provided by the Virginiaand Maryland Departments of Transportation. Truck rollover accidents are a relatively rare event here: Only five of the 14 Virginia ramps had two rollovers for a four-year period, and only one ramp of 27 in Maryland had more thanone rollover in a five-year period. This interchange in Maryland experienced a total of 15 rollovers with six on one specific ramp. The interchange was recently redone; consequently, the high rollover ramp is no longer a truck rollover problem site.

Alternative Concepts of a Truck Warning System

Ideally, an automatic truck warning system would detect a truck's weight and center of gravity sufficiently in advance of a curved ramp, "know" the curvature and superelevation of the ramp, and warn the truck driver to reduce speed to a level below the vehicle's rollover threshold speed. Given this general functional requirement, the study team postulated at least two detection/warning system concepts.

The first concept is an inroad detection-warning system that depends on an "intelligent highway." In this system, a detector or detectors placed in or along the road identifies the truck and its relevant parameters--speed, weight -- and a warning device -- an activated sign and/or flashing beacon -- is positioned in advance of the curved ramp. A controller receives the signal from the detector(s), processes the information according to an algorithm that determines if the truck's speed may cause a rollover, and transmits a signal to activate the warning device if the truck's speed is equal to or greater than the rollover threshold speed.

The second concept is an invehicle detection/warning system that depends on an "intelligent vehicle." At the start of each trip, the driver enters into an onboard computer information on the vehicle configuration -- number of trailers, trailer type -- cargo type and weight, and load distribution. The truck's speed is continuously monitored from a sensor on the tractor's drive axle, and the sensor's data is processed through the onboard computer. At each curved ramp or ramp with a history of rollover accidents or with a degree of curvature and superelevation that have been found to be associated with truck rollover, a transponder transmits the ramp geometrics data--ramp radius and superelevation -- to the truck. This radio signal is processed in the onboard computer. If a rollover is possible, an alarm signal or recorded message warns the driver to reduce speed.

Several factors make this latter concept highly appealing. The system determines the possibility of rollover from precise, accurate information about the truck's configuration, cargo type, weight, and speed. It provides information to the truck driver through invehicle displays as opposed to via an external device that might go undetected. Also, the ease and low cost of transponder installation allows agencies to install the system at most ramps.

However, this system is not currently available. Therefore, the study team focused on the inroaddetection/warning system.

Inroad Detection/Warning System Design Requirements

An inroad detection/warning system would include detector hardware, a controller for processing the electronic data, and a warning system. The requirements identified by the study team for each of these components are discussed below.

Detection system
For an inroad detection/warning system to operate effectively, it should capture certain vehicle parameters as a minimum -- vehicle type, such as truck or nontruck; speed and deceleration profile; and weight. Ideally, it should also be able to detect the truck's center of gravity; however, this is not possible for an in-road detection system. These parameters can be determined using various roadway detection systems.

  • Vehicle type identification.

Trucks can be identified and classified using either an inductive loop, a piezoelectric sensor, or a combination of the two systems coupled with a controller to process the electrical charges. When trucks pass over these sensors embedded in the pavement, they establish a vehicle charge or voltage profile which is then matched with existing FHWA data base profiles to classify vehicle type correctly.

Because tanker trucks have a lower rollover threshold than box trailer trucks of the same weight (see table 1), thesystem must be able to distinguish between these. Since tanker trucks are typically lower than box trailer trucks, avehicle height sensor can be used for making this distinction. Commercially available height detectors use amicrowave-based radar beam as an "electric eye" to detect a vehicle within the beam angle. By adjusting theheight of the detector above the pavement and properly angling the beam, this device can be used to detect trucksabove or below a threshold height. Tanker trucks are typically 3.4 meters (11 feet) or lower; this height should beestablished as the threshold value for distinguishing between a box trailer and tanker truck.


The sensor can detect trucks above or below a threshold height.


  • Truck speed detection.

The speed at which a truck is traveling at a specific point on a ramp is the most important variable in determiningits rollover potential. Therefore, accurate and reliable truck speed detection must be a prime feature of the system.If the truck speed is detected too early, the assumptions regarding the truck's speed profile, based on truckdeceleration rates, may not be accurate. On the other hand, if speed is detected too close to the curve, activationof a warning sign may not serve as a sufficient warning. Therefore, sensors for detecting truck speed must beplaced carefully and appropriately.

The speed of a vehicle can be determined using a pair of either embedded inductive loop detectors or piezosensors. A controller is needed to process the electrical charges and determine the speed. Thus, the same detectorhardware used for truck classification can, when properly arranged, also be used to determine the truck's speed.


Truck speed can be determined using a pair of embedded inductive loop detectors or peizo sensors.


Another desirable system feature would be its ability to determine the truck deceleration profile. Although a truckmay be traveling faster than the calculated rollover threshold speed at a point upstream of the curved section, itmay be decelerating at a certain rate that would bring it below the critical speed by the time it reaches the point ofcurvature. A speed deceleration profile can be determined by installing two-point speed detection systems.

  • Vehicle weight measurement.
  • The weight of a truck -- a useful indirect variable in determining the truck's rollover threshold -- can be obtainedby using commercially available weigh-in-motion (WIM) systems. These weight systems use a combination ofinductive loop and piezo sensors to provide electrical charges to a controller that is programmed to calculatevehicle weight. Some WIM systems can measure truck weight at an accuracy of 2 percent of the true weight fortrucks traveling up to 103 kilometers/hour (70 miles/hour).

    An electronic controller is needed to accept the electrical inputs from the detection, process the charges according toa prescribed logic for identifying a truck that is exceeding the rollover threshold, and send a signal to activate thewarning device. The controller should be housed in a cabinet; its electricity should be drawn from the nearestexisting source. The system should have a built-in capability to test each of the components and the system as awhole. Maintenance personnel could access this feature through switches provided in the controller.

    Warning device

    The following two alternative devices are suggested to warn the driver:

    • Option 1 -- static warning sign with yellow beacons. This sign would consist of a standard static truck rolloverwarning sign, flashing yellow beacons which would be activated by the controller if the system detects a potentialrollover, and a supplemental message explaining the flashing beacon.
    • Option 2 -- static warning sign with supplemental message. This sign would consist of a static truck rolloverwarning sign as in option 1 but without the flashing beacons. Instead, the sign would have a supplemental fiberoptic sign carrying the message "TRUCKS REDUCE SPEED." This message would appear only if a truck is at orexceeding the rollover threshold speed.

    A major drawback of the first option is its potential for tort liability. The device only warns a driver when itdetects a potential rollover. Consequently, if the system fails to detect a potential rollover and that vehicle does rollover, the agency could be held liable for not providing the expected warning. Under the second option, however, thedriver is not expecting the sign to activate unless the driver regularly drives the route and has "tested" the system.Thus, if the system fails to detect a potential rollover and does not activate the sign, the driver still receives thestandard warning from the static sign.


A speeding truck activates the warning sign.
A slower truck does not activate the sign.


  • System Design and Operation

    Upon selecting and determining the components of the inroad detection/warning system, the study team next selectedsites for system design and development. In consultation with Virginia and Maryland transportation representatives,three ramp sites were selected for preparation, installation, and evaluation. Two of the sites were dual-lane exits sothe system was designed for both lanes.

    In the system, detection stations 1 and 2, which are loop-piezo-piezo configurations, provide weight, vehicleclassification, and vehicle speed data to the programmable controller. If the vehicle is classified as a truck, the twoweights are compared and the heavier weight used. Also at station 2, a height detector determines if the truck is lessthan 3.4 m (11 ft); if so, it is classified as a tanker truck. A rollover threshold value is assigned to the truck based onits weight and tanker/nontanker classification, using the data programmed into the controller.

    Data from all stations are recorded and retained in the controller for a specified period. The data can bedownloaded to a microcomputer at the controller site or transferred to a microcomputer in a central office over acommunication link.

    System Costs and Benefits

    Based on plans, specifications, and cost estimates prepared for three installations on the Capital Beltway, the studyteam found that system costs at a single-lane ramp would be nearly $100,000 and for a dual ramp about $160,000.These costs include engineering design, materials, installation, and annual maintenance and operations costs. Thefinal estimated system cost did not include controller modification cost by the manufacturer, since this was aone-time cost for the three sites. It is assumed that if the system is installed at a significant number of locations, themodification costs would be amortized.

    The benefits from this automatic warning system are a reduction in truck rollover accidents and in the associatedcosts. These costs are the dollar values assigned to the fatalities, injuries, vehicle property damage and cargo loss,possible damage to the highway facility and appurtenances, motorist delays, and traffic control and clean-up causedby the accident.

    Truck accidents are costly, especially if hazardous cargo is involved. The study of truck accidents on urbanfreeways determined that the average total cost of a truck accident is $13,274. (4) An analysis of truck accidents onthe Capital Beltway established a cost per accident. (5) These costs were $1,200,000 per fatality, $13,650 per injury,and $2,425 for property damage. Using these costs and applying the observed distribution of accidents and accidentseverity on the Beltway for 1986-87, a value of $15,470 per truck accident was developed. This value did not includedelay or clean-up costs. It is likely that both of these estimated accident values -- $13,274 and $15,470 -- are less thanthe actual average costs of a truck rollover accident. A more likely average estimate is $20,000, with a significantprobability that such an accident will result in a fatality.

    Conclusion: Cost-Effectiveness Analysis

    Table 2 - Required Rollover Accident Reduction for System Cost-Effectiveness

    No. of Rollover Accidents
    All Accident Costs ($) 1-Lane System
    @ 109,000*
    @ 172,000*
















    *Installation costs plus $1,000 per year for 10 years for maintenance

    One way to assess the cost effectiveness of an automatic truck warning system is to establish how many accidentswould have to be eliminated by the system to make it pay for itself. Table 2 provides the results of this analysis.Increments of total accident costs ranging from the estimated average costs of $20,000 to $1,000,000 are listed withthe number of accidents that would have to be eliminated by a one- or two-lane system.

    Obviously, the cost effectiveness of this system is very much dependent on whether it prevents a high-costrollover accident -- an event that is relatively rare. There were 12 rollover accidents at seven ramps in Virginia over afour-year period. Linear extrapolation of this frequency rate reveals that there could be an average of 4.25 accidentsper ramp for those seven ramps in a 10-year period. It thus appears from this simplistic, but reasonable, analysis thatan effective automatic truck warning system could be cost effective if applied at ramps with a history of truck rolloveraccidents of at least one every five years.


    (1The Effect of Truck Size and Weight on Accident Experience and Traffic Operations, Publication No. FHWA-RD-80-137, Federal Highway Administration, Washington, D.C., July 1981.

    (2Impact of Specific Geometric Features on Truck Operations and Safety at Interchanges, Publication No.FHWA-RD-86-057, Federal Highway Administration, Washington, D.C., August 1986.

    (3) P.L. Olson, P.S. Fancher, Z. Bareket, D.S. Gattle, T. Aoki, T. Sato, E.C. Traube, and L.S. Pettis. An Evaluation of Freeway Ramp Speed Advisory Signs for Trucks. Report No. UMTR1-91-40, University of Michigan Transportation Research Institute,October 1991.

    (4) B.L. Bowman and H.S. Lum. "Examination of Truck Accidents on Urban Freeways," ITE Journal, October 1990, pp. 21-26.

    (5Washington Bypass Study: Technical Memorandum No. 3, Traffic, Bellomo-McGee Inc., Vienna, Va., April 1990.

    For more detailed information about the FHWA study "Feasibility of an Automatic Truck Warning System," contact:

    Howard H. Bissell

    Information and Behavioral Systems Division

    Office of Safety and Traffic Operations R&D

    Federal Highway Administration

    6300 Georgetown Pike

    McLean, VA 22101-2296

    Phone: (703) 285-2428

    Fax: (703) 285-2379

    Hugh W. McGee, P.E., is a principal of Bellomo-McGee, Inc. (BMI), a transportation and traffic engineering firm located in Vienna, Va. Dr. McGee has been involved in highway and traffic engineering research for 23 years and has conducted numerous projects for the Federal Highway Administration. Dr. McGee holds a bachelor's degree, master's degree, and doctorate in civil engineering from Pennsylvania State University.

    Rodney R. Strickland is an electrical engineering technician at BMI. In this capacity, he provides technical support in the area of signal, lighting, and communications design. Mr. Strickland has a bachelor's degree in engineering technology from Purdue University.