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FHWA Highway Safety Programs

Summary of Results

Table 17 shows the recommended speed limits yielded by each speed limit setting method and the actual speed limit enacted by the road authority for both case studies are shown below.

Table 17. Recommended Speed Limits for the Case Studies

 

Eldron Boulevard, Florida

State Route 67, California

Actual Speed Limit

40

55

Illinois DOT

40

55

Northwestern

45

55

USLIMITS2

40

55

Optimal Speed

45

50

Safe System Speed

30

50

With the exception of the safe systems approach, the recommended speed limit from each of the methodologies used are within 5 mph of each other. On the one hand, this suggests an inter-method consistency that is reassuring. However, it needs to be remembered that these are only two specific examples, and this consistency may not endure in other cases. In fact, the optimal speed and the safe systems approaches are known to produce results that have a more pronounced difference from the other methods in certain situations. This is perhaps not surprising since the Illinois DOT method, the Northwestern method, and USLIMITS2 all start from the 85th percentile speed.

As expected, the safe speed approach resulted in speed limits that are at the low end of the range. This becomes very apparent in the urban case on Eldron Avenue, where the potential for more frequent right-angle crashes requires a more dramatic decrease in operating speeds to be consistent with the zero tolerance for injury-producing crashes.