Speed Impacts of Traffic Calming Measures
(standard deviations in parentheses)

Sample Size

85th Percentile Speed Afterward

Average Change in 85th Percentile Speed

Average % Change

12' Speed Hump

179

27.4 mph
(4.0 mph)

-7.6 mph
(3.5 mph)

-22%
(9%)

14' Speed Hump

15

25.6
(2.1)

-7.7
(2.1 mph)

-23
(6)

22' Speed Table

58

30.1
(2.7)

-6.6
(3.2)

-18
(8)

Longer Table (>22')

10

31.6
(2.8)

-3.2
(2.4)

-9
(7)

Raised Intersection

3

34.3
(6.0)

-0.3
(3.8)

-1
(10)

Traffic Circle

45

30.3
(4.3)

-3.9
(3.2)

-11
(10)

Narrowing

7

32.3
(2.8)

-2.6
(5.5)

-7
(22)

Choker

5

28.6
(3.1)

-2.6
(1.3)

-14
(4)

Half Closure

16

26.3
(5.2)

-6.0
(3.6)

-19
(11)

Diagonal Diverter

7

27.9
(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 Size

Average Change in Volume

Average % Change

Choker

5

-392 vehicles per day
(384 vehicles per day)

-20%
(19%)

Full Closure

19

-671
(786)

-44
(36)

Half Closure

53

-1611
(2444)

-42
(41)

Diagonal Diverter

27

-501
(622)

-35
(46)

Other Volume Control

10

-1167
(1781)

-31
(36)

Safety Impacts of Traffic Calming Measures
( U.S. Experience)

Number of Observations

Average Number of Collisions

% Change in Collisions

12' Speed Hump

49

2.7

2.4

-11%

14' Speed Hump

5

4.4

2.6

-41%

22' Speed Table

8

6.7

3.7

-45%

Traffic Circle
(w/o Seattle )

17

5.9

4.2

-29%

Traffic Circle
(w/ Seattle )

130

2.2

0.6

-73%

All Measures

w/o adjustments

192

2.6

1.3

-50%*

w/ adjustments

42

3.8

3.0

-21%**

* Significant at 0.001 probability level
** Significant at 0.04 probability level