Network-wide Performance and Modelling
When a road network has been Star Rated, the Star Rating model allows us to estimate the benefit of numerous network-wide treatments. For example modelled scenarios might include the impact of the following on Star Ratings and, importantly for business cases, the impact on expected deaths and serious injuries:
- The impact of improving road condition and delineation to adequate across the road network – modelling the impact of maintenance-type countermeasures on road safety
- The impact of replacing ramped end vehicle restraint system terminals with passively safe terminals
- The impact of raised profile markings on roads where they are currently absent
- The impact of changing the speed limit on all rural single carriageway roads
- The impact of adding central hatching to all single carriageway roads to reduce head on risk
- The impact of adding crossing facilities for pedestrians where crossing is likely.
The opportunities for modelling mass actions at the network level are vast and this can help to identify opportunities to optimise the impact of various programmes on road safety, for example maximising the casualty reduction potential of routine road maintenance activities.
Analysis of this type helps to prioritise cost-effective road safety treatments that can be made across a road network, rather than simply at historic crash locations. This approach enables road authorities to substantially reduce the number of deaths and serious injuries on their road network in the medium-long term.
Also link to local roads where we say you can do network wide modelling …
Prioritisation
Where an iRAP survey has been completed across a whole road network, it is possible to use the Star Ratings as part of the intelligence base for prioritisation. Typically, RSF recommends using both the Star Rating (where this is low it indicates there may be an aspect of the road layout or operation that could be improved) and Fatal and Serious Collision Density (where this is high this indicates that there may be good returns on investment).
National Highways used this approach for identification of the routes to be treated under the National Programme.
Where the Star Ratings are not available, RSF uses an indicative Benefit to Cost Ratio (BCR) generated through Crash Risk Mapping to identify priority routes for further survey and analysis.
Road safety indicators
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.
Embedding iRAP into policy and approach
Much of iRAP’s work with National Highways and Welsh Government is to support the use of iRAP and the Route Review Tool in developing schemes that will be effective in reducing death and serious injury on these networks. However, without underpinning processes, policies and standards changing, there will be no difference in practice. So RSF works to identify opportunities within current funding streams, project delivery and decision making to embed iRAP and therefore Safe System delivery. An example of this is the collaborative development of the CAVE tool with National Highways which translates Route Review Tool outputs into the necessary inputs for Green Book compliant transport appraisal.
