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Publication Number:      Date:  Spring 1994
Issue No: Vol. 57 No. 4
Date: 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.

Static warning sign and an activiated warning sign used together.Yellow line.

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. (2, 3) 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


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)

Comparison of Rollover Thresholds.

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.

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.

Sensor detects trucks above or below a threshold height.

Yellow line.

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.

Pairs of inductive loop detectors can determine truck speeds.

Yellow line.

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.




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