1. Best practices for reducing speed
The role of speed limits and low-cost physical
changes to the road environment
Rune Elvik, Institute of Transport Economics, Oslo,
Norway (re@toi.no)

2. Effects of changes in speed limits on the mean speed of traffic
15.0
Dotted straight line = line of
proportionality = equal
percentage changes
10.0
Change in the mean speed of traffic (km/h) - vertical line
5.0
0.0
-40 -30 -20 -10 0 10 20 30
-5.0
-10.0
-15.0
y = -0.0058x2 + 0.2781x - 0.2343
R² = 0.5404
-20.0
-25.0
Change in speed limit (km/h) - horizontal line
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3. Speed limits are effective
 The speed of traffic almost always changes in the same
direction as the speed limit (up and up, down and down)
 The changes are not strictly proportional – the speed of
traffic changes less than the nominal change of speed
limit
 Reductions of speed limits have larger effects than
increases of speed limits
 The effects of changes in speed limits can be increased
by introducing supplementary measures:
 Enforcement (automatic or by police officers)
 Physical measures on the road
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4. Low-cost physical measures
 Speed humps
 Cheap, effective and require minimal maintenance
 Need to be placed densely to keep speed down on longer sections
 Raised pedestrian crossings
 Have similar effects as humps, but make crossing the road easier
 Street narrowing – broader sidewalks
 Also make crossing the road easier, but reduces its capacity
 Chicanes – road narrowings in zigzag pattern
 Are less effective than humps if the street has to remain wide
enough for two cars to be able to pass without one of them having
to stop and wait for the other
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5. Effects of physical measures
Percentage change in the number of accidents
Best 95% confidence
Accident severity Types of accidents affected estimate interval
Speed humps
Injury accidents All accidents on roads with speed humps -41 (-57; -34)
All accidents on roads nearby roads with speed
Injury accidents -7 (-14; -0)
humps
Raised junctions
Injury accidents Accidents at junctions +5 (-34; +68)
Property damage only accidents Accidents at junctions +13 (-55; +183)
Rumble strips in front of junctions
Injury accidents Accidents at junctions -33 (-40; -25)
Property damage only accidents Accidents at junctions -25 (-45; -5)
Unspecified Accidents at junctions -20 (-25; -5)
Speed zones
Injury accidents All accidents -27 (-30; -24)
Property damage only accidents All accidents -16 (-19; -12)
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