- Based on the free-flow speed data collected for a 24-h period
at the experimental and comparison sites in 22 States, posted
speed limits were set, on the average, at the 45th percentile
speed or below the average speed of traffic
- Speed limits were posted, on average, between 5 and 16 mi/h
(8 and 26 km/h) below the 85th percentile speed.
- Lowering speed limits by 5, 10, 15, or 20 mi/h (8, 16, 24,
or 26 km/h) at the study sites had a minor effect on vehicle speeds.
Posting lower speed limits does not decrease motorist's speeds.
- Raising speed limits by 5, 10, or 15 mi/h (8, 16, or 25 km/h)
at the rural and urban sites had a minor effect on vehicle speeds.
In other words, an increase in the posted speed limit did not
create a corresponding increase in vehicle speeds.
- The average change in any of the percentile speeds at the
experimental sites was less than 1.5 mi/h (2.4 m/h), regardless
of whether the speed limit was raised or lowered.
- Where speed limits were lowered, an examination of speed distribution
indicated the slowest drivers (1st percentile) increased their
speed approximately 1 mi/h (1/6 km/h). There were no changes on
the high-speed drivers (99th percentile)
- At sites where speed limits were raised, there was an increase
of less than 1.5 mi/h (2.4 km/h) for drivers traveling at and
below the 75th percentile speed. When the posted limits were raised
by 10 and 15 mi/h (16 and 24 km/h), there was a small decrease
in the 99th percentile speed.
- Raising speed limits in the region of the 85th percentile
speed has an extremely beneficial effect on drivers complying
with the posted speed limits.
- Lowering speed limits in the 33rd percentile speed (the average
percentile that speed were posted in this study) provides a noncompliance
rate of approximately 67 percent.
- After speed limits were altered at the experimental sites,
less than one-half of the drivers complied with the new posed
limits.
- Only minor changes in vehicles following as headways less
than 2s were found at the experimental sites.
- Accidents at the 58 experimental sites where speed limits
were lowered increased by 5.4 percent. The level of confidence
of this estimate is 44 percent. The 95 percent confidence limits
for this estimate ranges from a reduction in accidents of 11 percent
to an increase of 26 percent.
- Accidents at the 41 experimental sites where speed limits
were raised decreased by 6.7 percent. The level of confidence
of this estimate in 59 percent. The 95 percent confidence limits
for this estimate ranges from a reduction in accidents of 21 percent
to an increase of 10 percent.
- Lowering speed limits more than 5 mi/h (8 km/h) below the
85th percentile speed of traffic did not reduce accidents.
- The indirect effects of speed limit changes on a sample of
contiguous and adjacent roadways was found to be very small and
insignificant.

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