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
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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.
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| 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
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FIGURE 1: Characteristics of Severe Grades
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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.
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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:
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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.
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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.
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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
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FIGURE 3: Institute of Transportation Engineers Guidelines
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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.)
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|
Example of weight specific speed (WSS) sign
5 AXLES OR MORE
| |
WEIGHT |
MAX SPEED |
| |
65000 - 70000 |
35 |
| |
70000 - 75000 |
25 |
| |
75000 - 80000 |
15 |
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FIGURE 4: Weight Specific Speed
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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:
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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
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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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