The co-existence of two systems of measurement affects road contractors. While doubtless contractors have lived with this mess for some decades it is hardly ideal and potentially can result in errors. The use of two systems is entirely because of inconsistent measurement policy by the Department for Transport (DfT) as summarised in the table below.
|Rules & regulations||Measurement units|
|Road and marking designs||metric|
|Road sign design||metric|
Given the current state of British traffic signs, drivers could be forgiven for thinking that Britain's roads are designed in imperial units. Of course this is not the case and roads are designed using metric units. This is illustrated by the following extract from Chapter 4 of DfT's Traffic Signs Manual.
Consistent with British engineering practice DfT sets rules for placing warning signs in round numbers of metres. Therefore it is very odd that DfT also only authorises the distances on those signs to be in yards or miles!
Similarly rules for road markings such as mini-roundabouts are given in metric only.
In this case the rules are specified in millimetres.
Design of Traffic Signs
Like rules for road markings, parking, etc, DfT's rules for designing traffic signs are metric. The following example from Schedule 7a of the Traffic Sign Regulations and General Directions 2002 illustrates this.
Note that while the design rule for the sign itself is given in millimetres, the information on the sign is in feet, inches and miles. The signage contractors need to convert map or survey data from metric to imperial in order to comply with the regulations.
Yards or metres
DfT has frequently used the excuse that old people might be confused by metric units – a rather patronising statement! – however they themselves frequently allow yards to be used to represent metres.
An example of this is given in the diagram above. Contractors for roadworks are expected to place warning signs of the lane closure at intervals of 100 metres, however the signs themselves indicate intervals of 100 yards. DfT evidently thinks that motorists will not be confused by the 100 metre intervals!