Emergency exits

METRICATION: 5% complete
	Metric	Non-metric
	Sign specifications in millimetres.	Distances shown in yards.
	Sign positioning in metres.	Distances shown in miles and fractions.

A fire in a tunnel can lead to a rapid accumulation of smoke, which can severely restrict visibility and breathing. Emergency exit signs, placed at regular intervals, help pedestrians find the nearest safe exit.

Vienna Convention on Road Signs and Signals

The Vienna Convention on Road Signs and Signals of 1968, of which the UK is a signatory nation, specifies standard diagrams for emergency exit signs, with distances in metres. There is no prescribed option to use yards instead of metres:

The signs G, 24a, G, 24b and G, 24c are examples of signs to indicate the direction and distance of the nearest emergency exits. In tunnels, they shall be placed at a maximum distance of 50 m apart and at a height of 1 to 1.5 m on the sidewalls.

Part I: Convention on Road Signs and Signals – Annex 1

Metric positioning

UK legislation, in common with legislation throughout most of Europe, specifies a maximum spacing between signs of 25 metres, rather than the original 50 metres.

Non-metric units

Emergency exit signs in road tunnels are clearly a matter of public safety, and should therefore be required to show distances in metres as per the Units of Measurement Regulations. However, these signs fall within the scope of road traffic signs. In most countries this distinction would be irrelevant, but in the UK, the Department for Transport still retains the use of imperial units for distances on regular road signs.

Unfortunately, the DfT has opted to treat emergency exit signs for pedestrians as regular road signs, rather than as an issue of public safety. As such, it has opted to require that they show distances in imperial units.

TSRGD Diagram 2711.1

The Traffic Signs Regulations and General Directions (TSRGD) stipulates that the distances shown on emergency exit signs should be in one of two formats:

• yards, or
• miles (rounded to the nearest ¼ mile)

No advice is given about the rounding of values in yards.

Oddly, the format dictated for each sign depends on the total distance between the two nearest exits, and not on the size of the individual distances shown.

	CURRENT SIGNS					METRIC SIGNS
Distance from exit		Exits up to 880 yards apart		Exits > 880 yards apart		All exits
300 m
350 m
400 m
450 m
500 m
550 m
600 m
650 m
700 m
750 m
800 m
850 m
900 m
950 m
1000 m
1050 m

When distances are required to be in miles and fractions, the step size is ¼ mile, which corresponds to about 400 metres. This is clearly inappropriately coarse for pedestrian signs that are placed every 25 metres, and results in the value shown only changing after every 16 signs.

Why metric signs are clearer than imperial signs

The following two examples illustrate how the different imperial formats are applied:

• 350 metres into an 800-metre tunnel, the two distances would be shown as 383 yards and 492 yards.
• 300 metres into a 900-metre tunnel, both distances would be shown as ¼ mile. This is because 300 metres rounded to the nearest ¼ mile is ¼ mile, and 600 metres rounded to the nearest ¼ mile is also ¼ mile.

CURRENT SIGNS
Which is the nearest exit?

Using metric signs, distances shown can match the positions of the signs, which are in round multiples of 25 metres. Clear, precise information leaves no doubt as to the distance and direction of the nearest exit.

METRIC SIGNS
Nearest exit is on the left

The TSRGD rules also imply that when the distance between exits is greater than 880 yards (about 805 m), all distances of 200 metres or less should actually be shown as 0 miles. This is because 200 metres rounded to the nearest ¼ mile is 0 miles.

In practice, the TSRGD rules regarding the use of miles tend to be ignored, and all distances, long and short, are shown in yards rounded to the nearest yard.

CD 352 Design of road tunnels, Design Manual for Roads and Bridges

The requirement for distances to be signed in yards, rather than the simpler and more obvious metres, means that distances shown have to be rounded to odd values.

It is also important to note that no other major country uses yards on road signs, and so any visitor from abroad finding themselves in the unfortunate position of seeking an emergency exit from a tunnel fire, will have the additional burden of decyphering the UK’s use of antiquated measurement units while trying to escape.

Benefits of metric signs

Switching to the use of metres on emergency exit signs will have the following benefits:

• No rounding of distance values needed,
• Signs will be compliant with UK safety regulations,
• Signs will be compliant with Vienna Convention on Road Signs and Signals,
• Universally understood measurement symbols,
• No one under 60 was taught in yards and miles at school, but everyone is familiar with distances in metres.