The Design Variance

One area of concern for the Washington DOT with the cities’ redevelopment plans was the inclusion of trees near the roadway, both within the median (Figure 104) and along the outside, particularly for a high-volume highway such as SR 99 with speeds in the range of 40 to 50 mi/hr.  Plantings this close to the roadway did not meet Washington DOT’s clear zone criteria.  There were also concerns about the trees obstructing the drivers’ view of pedestrians.  

Even though clear zone is not one of FHWA’s 13 controlling criteria requiring a formal design exception, the Washington DOT followed a similar approach.  The existing crash problem along SR 99—with both high crash rates and severities—was a major motivation for improvements for both the DOT and the cities, so exceptions to any design criteria that could affect safety were carefully evaluated.  Working with the cities, the DOT developed and implemented several measures to monitor and mitigate the potential adverse safety impacts of the proposed designs.