The adequacy of fire safety measures in road tunnels has been questioned in recent years, following a series of major incidents in trans-alpine tunnels and this has prompted a re-appraisal of the fire safety philosophy for these structures.

However, opinions have varied as to what constitutes ‘best practice’ for tunnel fire safety and consequently, there has been a significant research effort (both by equipment manufacturers and by independent organisations) aimed at answering this question.

In addition to major advances in passive fire protection technology (e.g. the addition of ‘anti-spalling’ polypropylene fibres to concrete), there has been a similar R&D effort in the fields of fire detection (e.g. improved linear heat detectors) and active fire protection (e.g. Marioff’s HI-FOG high-pressure water mist system, see left).

Following a fire risk assessment of the 2.1 km, twin-bore, A303 Stonehenge Tunnel an active fire suppression was proposed, based on two main considerations:

	An analysis of forecast traffic levels
	The rural location (i.e. remote from a full-time fire station)

However, this proposal raised additional issues relating to the safe integration of such systems within the overall emergency procedures for tunnels:

	Maintenance and testing
	False discharges
	Operation on a fire
	Effectiveness on a fire in a tunnel
	Water on a carriageway
	Effect of water discharge during evacuation
	System does not discharge as expected
	Environmental and health issues

Halcrow’s review of the available fire suppression technologies included a quantitative assessment of the above risks, recommendations for further research to answer unresolved items and a whole-life cost estimate comparison of the various system options.