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 Geometric Design: Theory and Concepts

 
Ascending Grades

Efficiency and safety govern the design of ascending grades. Research has shown that the frequency of collisions increases dramatically when vehicles traveling more than 10 mph below the average traffic speed are present in the traffic stream. This 10 mph differential is, therefore, a bounding value in the design of ascending grades.

Research has also shown that most passenger cars are essentially unaffected by grades below 4-5%. Large commercial vehicles and recreational vehicles, on the other hand, are extremely sensitive to changes in grade. Design engineers do have some basic guidelines regarding the maximum upgrade for certain design speeds. Some of these recommended values are tabulated below (AASHTO, 1994). Note that these maximum upgrades are tolerable but not desirable, so they should not be used as targets for design.

Design Speed (km/h) Maximum Grade (%)
50 7-8
60 intermediate
80 intermediate
110 5

Let’s return to our discussion of the speed differential. For each grade, there is a critical length at which the design vehicles (trucks, RVs) will obtain the 10 mph differential. The figure below can be used to find the critical length for some common grades (AASHTO, 1984).

Graph of speed reduction based on percent upgrade and grade length.

As long as the length of your ascending grade is below the critical length, you will be able to maintain a reasonable level of safety, and large vehicles will not aggravate the traffic flow.

The general design process is this: design your roadway so that the ascending grades achieve the necessary change in elevation while not violating the maximum grade guidelines and not reducing the speed of trucks to more than 10 mph below the traffic's running speed. This can be done any number of ways, including a stepped approach with level sections between grades.

While the standards above should be the design goal, it is not always economically or physically possible to meet them. In these cases, climbing lanes may relieve some of the excess restriction. Climbing lanes are extra lanes that are reserved for slow vehicles. They allow faster vehicles to overtake slow vehicles safely and therefore increase the level of service for the highway. According to AASHTO, a climbing lane can be justified if all three of the criteria below are satisfied.

1. Upgrade traffic flow rate in excess of 200 vehicles per hour.
2. Upgrade truck flow rate in excess of 20 vehicles per hour.
3. One of the following conditions exists:

  • A 15 km/h or greater speed reduction is expected for a typical heavy truck.
  • Level-of-service E or F exists on the grade.
  • A reduction of two or more levels of service is experienced when moving from the approach segment to the grade.

Climbing lanes are becoming more and more common on two-lane highways. They are rarely used on multilane and divided-multilane highways, because these roadways currently accommodate the casual passing of slow vehicles.