INDIRECT EFFECTS ON NEARBY ROADS

Speed, headway, and accident data were collected at five selected highway sections located either contiguous to or adjacent to the experimental sites where speed limits were altered. Speed limits on the experimental sections were lowered at three sites and raised at two other sites. The purpose of these observations was to examine the indirect effects that speed limit changes may have on surrounding roadways. A summary of the effects for selected speed statistics is shown in table 12. The results in table 12 are typical of the other the other driver behavior measures examined in this study. A discussion of indirect effects at three additional sites is included in the next section of this report.

The first site listed in table 12 is a small urban area roadway with a posted speed limit of 35 mi/h (56 km/h). The site is located between 2 experimental sites where the speed limits were lowered from 50 to 45 mi/h (81 to 72 km/h). The next 2 sites listed in table 12 are located in a small city at each end of an experimental section where the speed limit was lowered from 45 to 35 mi/h (72 to 56 km/h).

The fourth site given in table 12 has a posted speed limit of 40 mi/h (64 km/h) and is contiguous to a rural experimental section where the speed limit was raised from 40 to 45 mi/h (64 to 72 km/h). The last site in table 12 is located in a small town adjacent to an experimental site where the speed limit was raised from 30 to 35 mi/h (48 to 56 km/h).

As can be seen by examining table 12, there were very few, if any, effects on the mean, standard deviation of speeds, or 85th percentile speeds at these locations after the speed limits were changed on the experimental sites. The examination of the speed distribution and vehicles following at headways of less than 2 s revealed similar results. The accident sample was too small to provide meaningful results.

Overall, there were very few driver behavior changes on any of the contiguous and adjacent roadways after speed limits were altered at the experimental sites.

      Table 12.  Indirect speed effects at contiguous sites.

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Posted                    Before            After          Differences
Speed  Experimental  Mean   Std. 85th  Mean   Std. 85th  Mean   Std.  85th
Limit     Section    Speed  Dev. Pct.  Speed  Dev. Pct.  Speed  Dev.  Pct.
mi/h      Change     mi/h   mi/h mi/h  mi/h   mi/h mi/h  mi/h   mi/h  mi/h
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  35     50 to 45     42.3   6.4   49   42.3   6.9   49    0.0    0.5    0
  45     45 to 35     47.1   8.4   55   47.1   7.3   55    0.0   -1.1    0 
  45     45 to 35     41.0   8.5   50   41.1   8.4   50    0.1   -0.1    0 
  40     40 to 45     47.0   4.8   52   46.5   4.9   51   -0.5    0.1   -1
  35     30 to 35     34.7   6.9   42   35.3   6.5   42    0.6   -0.4    0
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1 mi/h - 1.61 km/h

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