20 TR News 166, May–June 1993

RESEARCH PAYS OFF

Full-Scale Arrester Bed TestingLeads to More Cost-Effective Design

Truck escape ramps, or arrester beds, are acommon vehicle safety feature on steep,sustained grades. Unfortunately, arresterbeds are located typically in mountainousareas where construction is costly and siteselection difficult.

Problem

The most common form of truck escaperamp is the gravel arrester bed, which slowsvehicles by displacing the aggregate orgravel in the bed. The gravel imparts a roll-ing resistance to the vehicle motion, allow-ing a controlled deceleration and shorterstopping distance.

The effectiveness of an arrester bed de-pends principally on its geometry and ag-gregate characteristics. The main designconsiderations are the length and depth ofthe bed and the gradation and shape of theaggregate. Shape is important: the aggre-gate should be well rounded so that it candisplace easily, allowing a vehicle to sinkinto the bed. Highly fractured aggregatetends to sustain a vehicle's weight, result-ing in a lower rolling resistance.

Arrester beds are typically situated at lo-cations where construction costs are high;thus, it is desirable to use aggregate with avery high rolling resistance to allow shorterbed lengths--which provides for sizable costsavings in areas where rock excavation orfills are required for construction.

The day-to-day maintenance of arresterbeds may also affect their performance. Theenvironment and traffic cause some arrester

bed characteristics to deteriorate over time.Regular maintenance of the beds by fluff-ing may enhance their performance.

Solution

The Arizona Department of Transportation(ADOT) sponsored research to conductinservice testing at four of its seven arresterbeds. The testing was performed to verifythe rolling resistance of the aggregate and toevaluate several maintenance techniques.ADOT had recently changed its aggregategradation and specifications when it beganthe study. The new grading was more ex-pensive to produce, but it was hoped that itwould afford a higher rolling resistance thanthe gradation that had been used.

To evaluate the in-place aggregate roll-ing resistance, 102 entries were performedwith an unloaded single-trailer truck withfive axles. The truck entered each of thefour arrester beds at two speeds: 45 and 65mph (Figure 1). A radar unit was posi-tioned to measure the change in vehiclevelocity at 1/20-second intervals. Thesevalues allowed for backcalculation of therolling resistance.

The effect of bed preparation was alsoevaluated. Three types of scarification wereevaluated: grading by dozer, motor grader,and mechanical rake. Between truck entries,maintenance personnel prepared the bedsusing the three techniques. The effect on thestopping distance was then analyzed. Afourth condition was also evaluated to de-termine if vehicles that entered an arresterbed in the tracks left by a previous vehicletraveled farther into the bed.

Application

The results of the research indicated thatthe revised aggregate specification exhib-ited an effective rolling resistance of 0.3;the design rolling resistance at the time ofthe study was 0.25. Using the effectiverolling resistance allows the arrester bed tobe reduced 180 feet in length.

Because all four arrester beds evalu-ated were designed with different grades,transitions, and depths of aggregate, itwas difficult to quantify precisely theeffect of aggregate depth. It was deter-mined, however, that the depth of theaggregate could be reduced from

TR News 166, May–June 1993 21

48 to 36 inches. ADOT is evaluating thepossibility of further reducing the depth to30 inches.

The results of this study demonstratedthat entry speed, bed preparation, andtracking significantly affect the distancethat a truck travels into an arrester bed.When trucks follow in the tracks of a pre-vious vehicle, their stopping distance mayincrease by 15 to 20 percent. Removingthese tracks by scarification was recom-mended; the method to be used is notsignificant.

Benefits

Using the effective aggregate rolling re-sistance and reducing the thickness of thearrester bed have resulted in a construc-tion savings of about $30,000 per site.The savings can be much higher (in thehundreds of thousands of dollars) at loca-tions where rock cuts or high fills are re-quired. The higher rolling resistance al-lows the length of an arrester bed to bereduced by 15 percent.

Not only were construction costs saved,but the need to replace the aggregate inone of the beds was eliminated. Beforethe testing, there was concern about theadequacy of the only bed with the "old"aggregate gradation. Test results indicatedthat the bed performed satisfactorily andcould remain.

For more information, contact Larry Sco-field, Manager, Transportation Research,Arizona Transportation Research Center,Arizona Department of Transportation, 7755Research Drive, Suite 106, Tempe, Arizona85284 (telephone 602-831-6165).

Readers interested in further informationon arrester beds are referred to NCHRP Syn-thesis 178: Truck Escape Ramps. The reportis available (price: $9.00; $6.75 for TRBaffiliates) from the Transportation ResearchBoard, Box 289, Washington, D.C. 20055(telephone 202-334-3213 or 3214).

Suggestions for "Research Pays Off" topicsare welcome. Contact Crawford F. Jencks,Transportation Research Board, 2101 Con-stitution Avenue, N.W., Washington, D.C.20418 (telephone 202-334-2379).