Drivers’ reactions to circumstances that frustrate or annoy them vary greatly and are hard to predict. We all have our theories on what makes the roads safer or more dangerous, but ultimately it is the collision and casualty figures that provide the answers.

Fortunately, there is good evidence from sources all over the world that 20mph is much safer than 30mph. For example, In Calderdale, a 20mph limit led to a 30% reduction in casualties over a 3 year period. (See also: Does a 20mph speed limit really improve road safety?)

The harm caused by drivers determined to overtake at any cost is outweighed by the careful driving of the many. One reason may be the reduction in maximum convergence speeds. At 30mph the convergence speed is 60 mph but on at 20mph it is only 40 mph, allowing much more time for safe decision making.

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