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 Descending grades pose an entirely new set of problems for the design engineer. Instead of worrying about reductions in speed, the engineer must be concerned about unbridled increases in speed. The potential consequences of runaway vehicles are evident when you consider a highly populated area that is located at the base of a long, steep grade. To avoid catastrophes created by runaway vehicles, runaway vehicle ramps are often designed and included at critical locations along the grade. The location of runaway ramps depends on the geometry of the roadway and the topography of the surrounding terrain. Logically, a ramp should exist before each turn that cannot be negotiated at runaway speeds. Ramps should also be placed along straight stretches of roadway, wherever unreasonable speeds might be obtained. In addition, the ramps should be located on the right side of the road whenever possible, because opposing traffic or other vehicles may not realize that the truck is in trouble and be able to yield in time. Runaway ramps are normally designed so that they can stop a truck moving at a speed of at least 80 mph. Note that this is only a minimum speed and extra ramp length should be provided if the potential for greater entrance speeds exists. The ramp should also be wide enough to service more than one vehicle at a time. There are several different ramp types that can be employed to stop runaway vehicles. Figure 1.0 illustrates the various types of ramps that are commonly used.
Figure 1.0: Common Runaway Vehicle Ramps Along with the runaway ramp considerations, the engineer should give some thought to including a slow vehicle lane on downgrades. Trucks often use their lower gears and crawl down descending grades, to minimize the use of their brakes. Including a slow vehicle lane will provide faster traffic with a safe path for overtaking slow vehicles, and the extra lane may also provide endangered vehicles with an escape route if they discover a runaway in their rear-view mirror.
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