Proposed statistical methods for comparing road traffic collision and casualty rates

One way that National Highways monitors the safety of the strategic road network is by calculating and monitoring road traffic collision and casualty rates. We do this by using traffic data and STATS19 data (road traffic collision and casualty records collated and managed by the Department for Transport).

We expect these rates to fluctuate from road to road, between different road types and from year to year. This is because, fortunately, road traffic collisions are quite rare given how many miles are driven on the network. But these fluctuations can make it difficult to compare safety on different roads or road types, or over time as they make it difficult to understand whether a particular difference is more than the fluctuation we expect.

To help us make these comparisons more robustly, we've developed some statistical methods that we propose can be used to calculate confidence intervals on road traffic collision and casualty rates. We've also developed methods that we propose can be used to carry out hypothesis tests to compare collision and casualty rates.

For transparency, and because we hope these methods will be useful to the wider road safety community as well as to National Highways, we've published the following document describing our proposed methods:

Statistical methods for comparing road collision and casualty rates: proposed approach

While we've rigorously and carefully developed these proposed methods, they're not yet finalised. Before establishing them as best practice across National Highways, we welcome feedback and advice. In particular, we're keen to hear from you if you uncover a problem with our proposed methods or can suggest an alternative approach which would be a substantial improvement.

Feedback can be given to the Network Analysis and Statistics group by the end of August 2022, after which the methods will be updated if necessary and finalised.