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 Geometric Design: Professional Practice

 
Descending Grades

The following excerpt was taken from the 1994 edition of AASHTO's A Policy on Geometric Design of Highways and Streets (pp. 268-276).

Where long descending grades exist or where topographic 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. An out-of-control vehicle generally is the result of an operator losing control of the vehicle because of loss of brakes either through overheating or mechanical failure, or failure to downshift at the appropriate time.

Specific guidelines for the design of escape ramps are lacking at this time. However, considerable experience with ramps constructed on existing highways has led to the design and installation of effective ramps that are saving lives and reducing property damage. Reports and studies of the existing ramps indicate that their operational characteristics are providing acceptable deceleration rates and affording good driver control of the vehicle on the ramp.

. . . Escape ramps generally may be built at any feasible location where the main road alignment is tangent. They should be built in advance of main line curvature that cannot be negotiated safely by an out-of-control vehicle and in advance of populated areas. Escape ramps should exit to the right of the main line. On divided multilane highways, where a left exit may appear to be the only feasible location, difficulties may be expected by the refusal of vehicles in the left lane to yield to out-of-control vehicles attempting to shift lanes.

. . . Speeds in excess of 130 to 140 km/h will rarely, if ever, be attained. Therefore, an escape ramp should be designed for a minimum entering speed of 130 km/h, a 140 km/h design speed being preferred. Several formulas and software programs have been developed to determine the runaway speed at any point on the grade. These methods can be used to establish a design speed for specific grades and horizontal alignments. The design and construction of effective escape ramps involve a number of considerations as follows:

  1. To safely stop an out-of-control vehicle, the length of the ramp must be sufficient to dissipate the kinetic energy of the moving vehicle.
  2. The alignment of the escape ramp should be tangent or of very flat curvature to relieve the driver of undue vehicle control problems.
  3. The width of the ramp should be adequate to accommodate more than one vehicle because it is not uncommon for two or more vehicles to have need of the escape ramp within a short time. . . .
  4. The surfacing material used in the arrester bed should be clean, not easily compacted, and have a high coefficient of rolling resistance. . . .
  5. Arrester beds should be constructed with a minimum aggregate depth of 0.6 m. Contamination of the bed material can reduce the effectiveness of the arrester bed by creating a hard surface layer up to 300 mm thick at the bottom of the bed. Therefore, an aggregate depth up to 1000 mm is recommended. . . .
  6. A positive means of draining the arrester bed should be provided to help protect the bed from freezing and avoid contamination of the arrester bed material. . . .
  7. The entrance to the ramp must be designed so that a vehicle traveling at a high rate of speed can enter safely. . . .
  8. Access to the ramp must be made obvious by exit signing to allow the operator of an out-of-control vehicle time to react, so as to preclude the possibility of missing the ramp. . .
  9. . . .A service road located adjacent to the arrester bed is needed so the wrecker and maintenance vehicles can use it without becoming trapped in the bedding material. . . .
  10. Wrecker anchors, usually located adjacent to the arrester bed at 50 to 100 m intervals, are needed to secure the tow truck when removing a vehicle from the arrester bed. . . .