Before a road tunnel you pass a plain blue circular sign showing the white number '30' with no red border or diagonal stripe. Shortly after leaving the tunnel, you pass an identical blue circle showing '30' but now with a single red diagonal line drawn through the number. What do these two signs together require of you?
- The first sign sets a mandatory minimum speed of 30 mph inside the tunnel unless it is unsafe to maintain it, and the second sign cancels that minimum from that point onward
- Both signs are advisory only, suggesting 30 mph as a comfortable cruising speed in good conditions, with no legal force either way
- The first sign fixes the maximum speed permitted inside the tunnel at 30 mph, and the second sign cancels that maximum, after which the previous limit resumes
- The first sign only applies to goods vehicles and coaches, telling cars nothing about their own permitted speed inside the tunnel
Why A? And why not the others?
Correct answer: A. The first sign sets a mandatory minimum speed of 30 mph inside the tunnel unless it is unsafe to maintain it, and the second sign cancels that minimum from that point onward
A plain blue circle is a positive instruction rather than a prohibition, and when it carries a bare number with no border or stripe it is the minimum speed limit sign: vehicles must not travel slower than the stated figure while it applies, except where safety genuinely requires dropping below it, such as queuing traffic, poor visibility, or a breakdown. The matching blue circle showing the same number struck through by a single diagonal line is the specific 'end of minimum speed limit' sign, cancelling that requirement rather than introducing a new maximum. The option treating both signs as merely advisory ignores that a blue circular sign is a mandatory instruction under UK traffic sign law, not a suggestion. The option describing the first sign as fixing a maximum speed confuses the minimum-speed sign with an ordinary red-circle maximum speed limit sign; the shape and colour code specifically distinguishes the two, and a red circle would be needed to impose an upper limit. The option restricting the instruction to goods vehicles and coaches invents an exemption that does not exist: a plain mandatory sign with no vehicle-symbol plate applies to all traffic unless a plate specifically narrows it, and no such plate is described here.
Source: GOV.UK, The Highway Code — Traffic signs (minimum speed limit and end of minimum speed limit signs); DfT Know Your Traffic Signs