Effectiveness

Speed Impacts of Traffic Calming Measures

(standard deviations in parentheses)
Sample Size85th Percenile
Speed Afterward
Average Change in
85th Percentile Speed
Average % Change
12' Speed Hump17927.4 mph
(4.0 mph)
-7.6 mph
(3.5mph)
-22%
(9%)
14' Speed Hump1525.6
(2.1)
-7.7
(2.1 mph)
-23
(6)
22' Speed Table5830.1
(2.7)
-6.6
(3.2)
-18
(8)
Longer Table (>22')1031.6
(2.8)
-3.2
(2.4)
-9
(7)
Raised Intersection334.3
(6.0)
-0.3
(3.8)
-1
(10)
Traffic Circle4530.3
(4.3)
-3.9
(3.2)
-11
(10)
Narrowing732.3
(2.8)
-2.6
(5.5)
-7
(22)
Choker528.6
(3.1)
-2.6
(1.3)
-14
(4)
Half Closure1626.3
(5.2)
-6.0
(3.6)
-19
(11)
Diagonal Diverter727.9
(5.2)
-1.4
(4.7)
0
(17)
Note: speeds are measured at midpoints between measures

Volume Impacts of Traffic Calming Measures

(standard deviations in parentheses)
Sample SizeAverage Change in VolumeAverage % Change
Choker5-392 vehicles per day
(384 vehicles per day)
-20%
(19%)
Full Closure19-671 vehicles per day
(786 vehicles per day)
-44%
(36%)
Half Closure53-1611 vehicles per day
(2444 vehicles per day)
-42%
(41%)
Diagonal Diverter27-501 vehicles per day
(622 vehicles per day)
-35%
(46%)
Other Volume Control10-1167 vehicles per day
(1781 vehicles per day)
-31%
(36%)

Safety Impacts of Traffic Calming Measures

(U.S. Experience)
# of ObservationsAverage # of Collisions
Without Measure
Average # of Collisions
With Measure
% Change
in Collisions
12' Speed Hump492.72.4-11%
14' Speed Hump54.42.6-41%
22' Speed Table86.73.7-45%
Traffic Circle
(w/o Seattle)
175.94.2-29%
Traffic Circle
(w/Seattle)
1302.20.6-73%
All Measures
w/o Adjustments1922.61.3-50%*
w/Adjustments423.83.0-21%**
* Significant at 0.001 probablity level
** Significant at 0.04 probability level

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