Table 1.

Vehicular occupation. The probability of impact (P) is calculated D3600 ÷ S1000 = T; TV = H3600; H ÷ 24 = P.

Road classS
Average speed (kph)
Minimum stopping distance plus 6 m vehicle length (m)
Time that each vehicle occupies length of road ‘D’ (sec)
No. vehicles/dayz (1 direction only)y
No. hours for which a point on the road is occupied each day
Probability of impact with or by a tree/branch
Trunk road (built-up area)48292.1719,20011.51/2.1
Trunk road (non-built-up area)64422.3615,50010.11/2.4
Principal road (built-up area)48292.1715,0009.01/2.7
Principal road (non-built-up area)64422.367,2004.71/5.1
Minor road (all classes)64422.361,4000.91/27
  • 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.