Speed cushions are an effective treatment because the wider wheelbase of larger vehicles such as trucks and buses means they straddle the vertical deflection, whereas the narrower wheelbase of passenger cars is directly affected by it. This is important because buses and trucks can be more adversely affected by vertical deflection through passenger comfort impacts and freight impacts. It’s also important to note that these larger vehicles generally demonstrate higher compliance with posted speed limits and make up a much smaller proportion of vehicles on most roads.
However, a common behavioural response is for drivers of cars to avoid the vertical deflection by steering around the cushion, either to the left or the right. This needs to be explicitly considered in the design. Cushion width, lateral placement, and spacing should be configured so that a driver would struggle to avoid the vertical deflection.
Speed cushions are a traffic calming feature for which the iRAP Road Safety Toolkit reports a Crash Reduction Factor in the order of 25-40% percent. This reflects their effectiveness when appropriately designed and applied in suitable traffic environments.
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