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Synthesis of Safety Research Related to Speed and Speed Management - Turner-Fairbank.. Page 10 ot=' 25 Table 2. Speed increases observed from raising speed limit from 55 to 65 mi/h mi/h km/h Brown et al. (1990) Freedman and Esterlitz (1990) Mace and Heckard (1991) Prefer, Stenzel, and Lee (1991 ) Parker (1997) 2.4 3.9 2.8 4.5 3.5 5.6 4-5 6-8 0.2-2.3 0.3-3.7 Finch et al. (1994) analyzed the changes in speeds from raising and lowering speed limits reported in a number of international studies and found that the change in mean traffic speed is roughly one-fourth of the change in the posted limit. SPEED LIMITS AND SAFETY Another way to examine the relationship between vehicle speed and traffic safety is to measure the effects of lowering or raising speed limits on the incidence and severity of crashes. Table 1 summarizes the results of studies of this type conducted in several countries. The table shows that crash-incidence or crash severity, or both measures, generally decline whenever speed limits have been reduced. Conversely, the number of crashes or crash severity generally increased when speed limits were raised, especially on freeways. Parker (1992) found little change in crashes on Iow and moderate speed roads in Michigan where speed limits were altered under the State's normal speed zoning process. For the 21 sites where the speed limit was increased, crashes decreased about 3 percent compared to sites not changed. Crashes also decreased approximately 2 percent at the 47 sites where speed limits were lowered. Neither change was statistically significant. Parker (1997) found no significant changes in total or injury crashes for the 98 sites where speed limits were altered in the 22 States. This should not be surprising since, as discussed in the previous section, there were little or no change in speed. Compared to sites not change, crashes increased on the average 7 percent at sites where the speed limits were lowered and decreased on the average 11 percent where the speed limits were increased. Table 3. Summary of the effects of raising or lowering speed limits. Reference Country Change Speed Limit Decreases 110 km/h to 90 km/h Nilsson (1990) Sweden (68 mi/h to 56 mi/h) 60 km/h to 50 km/h Engel (1990) Denmark (37 mi/h to 31 mi/h) · 100 km/h to 80 km/h Peltola (1991) UK (62 mi/h to 50 mi/h) Results Speeds declined by 14 km/h Fatal crashes declined by 21% Fatal crashes declined by 24% Injury crashes declined by 9% Speeds declined by 4 km/h Crashes declined by 14% http://www.ntl.bts.gov/ntl/DOCS/speed/speed.htm 07/19/2000