Vehicular occupation. The probability of impact (P) is calculated D3600 ÷ S1000 = T; TV = H3600; H ÷ 24 = P.
| Road class | S Average speed (kph) | D Minimum stopping distance plus 6 m vehicle length (m) | T Time that each vehicle occupies length of road ‘D’ (sec) | V No. vehicles/dayz (1 direction only)y | H No. hours for which a point on the road is occupied each day | P Probability of impact with or by a tree/branch |
|---|---|---|---|---|---|---|
| Motorway | 113 | 102 | 3.25 | 30,450 | 27.5x | 1/1 |
| Trunk road (built-up area) | 48 | 29 | 2.17 | 19,200 | 11.5 | 1/2.1 |
| Trunk road (non-built-up area) | 64 | 42 | 2.36 | 15,500 | 10.1 | 1/2.4 |
| Principal road (built-up area) | 48 | 29 | 2.17 | 15,000 | 9.0 | 1/2.7 |
| Principal road (non-built-up area) | 64 | 42 | 2.36 | 7,200 | 4.7 | 1/5.1 |
| Minor road (all classes) | 64 | 42 | 2.36 | 1,400 | 0.9 | 1/27 |
↵z Transport Statistics Great Britain (1997).
↵y For the purpose of assessing the probability of impact, the total number of vehicles occupying all lanes of a motorway traveling in a single direction must be considered.
↵x Due to the sheer volume of traffic using motorways and the need to consider stopping distances, the vehicular occupation period is theoretically greater than 24 h.