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Road Management & Engineering Journal
Copyright © 1997 by TranSafety, Inc.
August 10, 1997
TranSafety, Inc.
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Truck Escape Ramps: Determining the Need and the Location
Appeals Court Reviews "Legal Duty" and "Discretionary Function" in Runaway Ramp Crash in Idaho
Effects of Aging on Older Drivers
Vision and Driving Performance in Older Drivers
Easy Ways to Use Waste Glass as Aggregate
Study Discussed Characteristics of Longer Combination Vehicles (LCVs) in Relation to Roadway Design



Truck Escape Ramps: Determining the Need and the Location

Truck escape ramps have been part of our highway system for well over 30 years. They are found in many different settings including the mountains, the suburbs, and even in small urban communities. The following summary reviews current practice used to identify the need and the appropriate site for truck escape ramps. The information presented here is taken from excerpts of the National Cooperative Highway Research Program (NCHRP) Synthesis 178 entitled "Truck Escape Ramps" (May 1992).

Background

The combination of heavy trucks and highway downgrades has long presented potentially dangerous conditions for truck drivers, other drivers on the road, and occupants of roadside property. The problem of runaway trucks generally results from brake failures which can arise for many different reasons. The inability of drivers to control vehicle speeds on downgrades is not only hazardous but it can also have costly consequences.

Recent statistics from the Federal Highway Administration 1989 report "Grade Severity Rating System (GSRS)--Users Manual" offer evidence of the magnitude of this problem. In one mountain state, one-sixth of the truck accidents were runaway downgrade accidents. A 1981 study for the National Highway Traffic Safety Administration (NHTSA) estimated that runaway truck incidents totalled 2,450 per year, incurring costs of nearly $37 million at that time. Of the total, 2,150 runaways were estimated to use escape ramps and incurred costs slightly over $1 million. The remaining estimated 300 crashes, vehicles not using ramps, cost close to $36 million. In half the runaway events involving trucks over 60,000 pounds, brake failure due to overheating was said to be the primary cause.

Current Truck Escape Ramp Statistics

As of 1990, TERs (truck escape ramps) numbered about 170 in the 27 states reporting them, three times as many as reported in 1970. While most are in western states, over 60 are in 12 states east of the Mississippi River. The states without escape ramps are primarily southern, midwestern or Great Plains states. Ramps already constructed report varying degrees of usage. But even rare usage can warrant ramp construction.

TER Characteristics

TERs are generally used in two situations: on long mountain grades in rural areas and on short steep hills likely to be in areas of dense traffic and development. Those located at the bottom of short steep hills are often in areas where there have been accidents involving fatalities or areas where serious property damage has occurred. TERs are likely to be found in locations that require a stop or slow-speed turn at the bottom of a grade.

Figure 1 lists characteristics of grades in selected states. Although the greatest amount of TERs are found on long downgrades in mountainous regions, heavily traveled interstates and the high volume of trucks forced to travel at low speeds through peak-hour traffic also cause problems.

Location Percent Grade Length (miles)
Pennsylvania
I-376
I-279
Stoops Ferry Rd.
Hulton Rd.
-
5
5.5
10.5
10
-
1.8
1.7
0.45
0.3
Idaho
Lewiston Hill
Whitebird Hill
-
6-7
7
-
7
7
Oregon
Siskiyou Summit
-
5 - 6.4
-
7
California
I-80
-
5 - 6
-
40
FIGURE 1: Characteristics of Severe Grades

Indicators

Determining where a TER is necessary involves several different considerations and these vary from state to state. A survey in the 1979 report, "State Practice and Experience in the Use and Location of Truck Escape Facilities" by R.W. Eck (Transportation Research Record 736), cited the principal influence as runaway accident experience. The results, summarized in Figure 2, show that many factors are considered in determining TER need, but the accident rate is cited almost twice as often as other factors.


Although current sources do not provide a consensus for guidelines for where TERs are needed, the 1990 edition of the American Association of State Highway and Transportation Officials (AASHTO) "A Policy on Geometric Design of Highways and Streets" referred to as the "Green Book" offers the following:

Where long descending grades exist or where topographic and location controls require such grades on new alignment, the design and construction of an emergency escape ramp at an appropriate location is desirable for the purpose of slowing and stopping an out-of-control vehicle away from the main traffic stream . . . Specific guidelines for the design of escape ramps are lacking at this time . . . [T]he principal determinations as to the need should be the safety of the other traffic on the roadway, the operator of the out-of-control vehicle, and the residents along or at the bottom of the grade.

The author notes that the AASHTO text on truck escape ramps is almost a verbatim copy of the 1979 FHWA (Federal Highway Administration) "Interim Guidelines for Design of Emergency Escape Ramps" (Technical Advisory T5040.10).

The Institute of Transportation Engineers (ITE) issued "Guidelines to Determine Need" in 1989. The ITE recommended practice states:

There are, however, many interrelated and not fully understood factors to consider when determining the need to provide a truck escape ramp . . . We suggest that engineering judgment be used when considering these factors (see Figure 3) until research can be used to establish a set of numerical warrants.

ITE Guidelines To Determine Need of TERs

Consider:

  • accident rates
  • relationship between horizontal alignment and operating speeds
  • potential severe accidents (e.g., high volumes of school buses)

Use Progressive Improvements Before Building Escape Ramps:

  • signing
  • speed controls
  • mandatory pull-off areas
FIGURE 3: Institute of Transportation Engineers Guidelines

When a location is designated as a possible TER site, the progressive improvement approach is recommended. It promotes better cost control because of gradual investment in determining the specific needs for a specific steep grade. The FHWA's Grade Severity Rating System (GSRS) may be a useful tool in the progressive approach because it calculates values for "Weight Specific Speed" (WSS) signs that alert the driver as to the maximum safe speeds on grades for vehicles of different weight. (See Figure 4.)

Example of weight specific speed (WSS) sign

5 AXLES OR MORE

WEIGHT MAX SPEED
65000 - 70000 35
70000 - 75000 25
75000 - 80000 15
FIGURE 4: Weight Specific Speed

Potential sites for WSS signs may be identified in several ways: observation, police reports on speed violations, maintenance records on guardrail or other hardware repairs, fire department responses to "hot brake" problems, and citizen complaints about speeding trucks.

Besides evaluating grade severity and determining downhill truck speed limits, GSRS can also be used to establish the need and location for truck ramp escapes, as its computer program has the option of calculating brake temperatures at 1\2 mile intervals along the downgrade. The calculated maximum safe descent speeds also provide a nonaccident method to substantiate traffic engineering improvements. The GSRS can be used to identify hazards before crashes happen and aid in determining the appropriate level of countermeasures.

Response to TER Criteria Survey

Part of this study involved surveying state transportation agencies regarding their current practices. The survey response showed several criteria influence state decisions to build a TER. Although accident experience was an influence in 21 states, it was not exclusive anywhere. Eleven states consider the GSRS, and it was the sole influence in two of these states. Engineering judgment was a factor in 24 states and the determining factor in two of them. Other factors in various combinations were: ramp location, smoking brakes, enforcement, truck speeds, signals, inspection/scale facilities, severity resulting in public opinion pressure that may influence building a ramp, and availability of right of way.

Site Selection for TERs

The criteria for site selection has been documented in many states. Some are guided by economics while others are concerned with the effect of weather conditions. The author states that the California Traffic Bulletin No. 24 "Design Guide for Truck Escape Ramps" (1986) and the ITE "Truck Escape Ramps" provide current considerations in the most detail. Figure 5 reports the site selection factors cited by state agencies in the TRR No.736 (Eck, R.W.) survey.

A 1982 FHWA survey, "The Development and Evaluation of a Prototype Grade Severity Rating System," concluded that:

The feasibility, type of design, and location of escape lanes are based primarily on engineering judgment . . . [E]ach segment presents a unique set of design requirements, dependent in part upon the following factors:

  • Nature of terrain along the segment
  • Degree of slope and roadway alignment
  • Availability of sights adjacent to the highway
  • Environmental impact
  • Logical site distance below the summit
  • Maximum potential speed of runaway trucks

Information obtained from all these reports suggests there is no clear consensus on current practices for determining when there is a need for a TER. Likewise, there are no clear guidelines for fixing specific locations for truck escape ramps. Further research can help improve future planning and practice.

Copyright © 1997 by TranSafety, Inc.



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