in the london area road safety is directly ensured by tfl if the road in question is part of the tfl road network and by the boroughs and the city of london for roads on their territories vision zero emphasises preventive measures that promote safe speeds street design vehicles and behaviours on roads as well as measures that reduce the severity of injuries post collision such interpolations are essential to support the computation of a five year average denominator eg population traffic trips as survey data can be missing between survey years