Reading BR187 part 2 written by Margaret Law, and noted that through her analysis of the effect of radiation on combustible materials - principally wood- she recommended that exposure and spacing between buildings should be controlled to ensure that the minimum radiation for pilot ignition of wood should not be exceeded.
She made the point that 10 minutes exposure would be required for ignition to take place and used statistics to show attendance times by county borough fire brigades were on average 4 minutes, well within the 10 minutes, thus providing a good safety margin.
This work still holds good today, and remains the basis for Approved Document B4.
The statistics were gathered in 1957- indeed County borough fire brigades ceased to exist on 1 April 1974 - I wonder if the statistics and safety margins still hold good? What would be the average attendance times in city centres 2008 as a result of the modernisation of the fire service?