Why are Design Exceptions Needed?
There is a broad range of reasons why design exceptions may be considered and found to be necessary. Some of these include the following:
- Impacts to the natural environment
- Social or right-of-way impacts
- Preservation of historic or cultural resources
- Sensitivity to context
- Sensitivity to community values
- Construction or right-of-way costs
The reason for a design exception may be a combination of several factors. For example, in a mountainous area, flattening the grades and lengthening vertical curves to achieve a vertical alignment that meets design criteria may have both severe environmental impacts and an exorbitant economic cost.
Even though there may be valid reasons for design exceptions, designers should be reluctant to design outside of accepted values. Understanding that the basis for the criteria is related to important performance as discussed above, it is reasonable to assume that any given design exception may also have the potential to adversely affect safety and traffic operations. A location where a design exception is being considered should therefore be thoroughly analyzed and the potential impacts understood before committing to the design exception
When the decision is made to go forward with a design exception, mitigation measures should be evaluated and, where appropriate, implemented to minimize the potential adverse impacts to the safety and operation of the highway.