iRAP Star Ratings are a useful lead safety indicator for the measurement of Safe Roads, typically the percentage of travel on 3-star or above roads is used. This has been used by National Highways as an indicator during RIS 1 and RIS 2. More recently, the notion of a decimal Star Rating has been identified as complementing the 3-star or above metric. Decimal Star Ratings give more granularity both the route and network level. Moreover, they help detect smaller incremental changes in Star Ratings that do not traverse the 3 star threshold, for example noting a route has increased from 2.1 stars to 2.9 stars.
In addition to these countermeasures, RSF has developed two further lead safety indicators available in the Route Review Tool. The first relates to the flow weighted average compliance which compares the 85th percentile speed along the route with the posted speed limit. The second relates strongly to the measurement of the Safe System, and this is the safety gap. This is the average flow weighted difference between the 85th percentile speed and the speed at which a given Star Rating level would be achieved (3, 4 or 5 star speed). What this tells us is how big the gap is between how fast people are travelling and how fast they would need to travel to be safe at a desired level of safety. The way to close the safe gap is either to reduce speeds, or to engineering upward – providing safer facilities along a road. The Government of Jersey has recently adopted this indicator in their Casualty Reduction Plan.
