Safe streets—challenging the principles

This paper examines the philosophy underpinning the process of designing urban streets in the UK. Engineers are trained to identify and design out failure modes, a model which serves them well in many fields. However, it is less suitable when considering road safety failures (accidents), where human factors are much more significant. When designing highways, engineers typically use simple mathematical models that appear to relate geometric criteria to accident risk, but these models are less rational than might be assumed. Because of this, engineers are generally unable to quantify the risk arising from deviating from highway norms. Engineers therefore fall back on the imprecise tool of the safety audit, which is the preserve of another specialist, and which has several flaws. Urban streets, despite their increased complexity, are based on similar mathematical models, which have led in many places to insensitive designs that have marginalised pedestrians. It is argued that a new evidence-based approach is needed to the design of urban streets that takes into account their multi-functional nature and the way in which users of the space modify their behaviour according to its layout and appearance. This new approach should be promoted via a comprehensive Manual for Streets, which would become the principal source document for all those involved in the design and maintenance of urban streets.