A short local A-road journey can still take longer than its mileage suggests. Great Britain’s local A-road average speed was 24.6 mph in the year ending March 2026, making it a useful planning benchmark rather than a prediction for a particular trip.
Calculate a baseline
Start with the mileage that will actually be covered on local A-roads. Divide that distance by 24.6 to estimate the journey time in hours at the published average, then convert the answer into minutes. Compare that baseline with the time already allowed. If the planned time is shorter, the gap is a sensible starting point for deciding how much extra time to add. This method simply translates the reported average into a time allowance for the distance being planned.
Keep the calculation limited to the local A-road portion. The published speed describes that road category across Great Britain; it is not an all-road average and should not be applied automatically to every mile of a mixed route. Separating the relevant mileage prevents the benchmark from appearing more precise or more widely applicable than the evidence supports. For the remaining mileage, retain the estimate already being used rather than substituting this benchmark.
Recognise the limits
The key limitation is scale. The figure is an annual average for local A-roads nationwide, while the evidence provided here gives no separate average for an individual road or departure. It therefore cannot tell a driver exactly how long one journey will take. The actual trip may be quicker or slower than the baseline, and the figure does not show by how much. Nor does it identify the likely direction or size of that variation.
This is also why there is no evidence-based fixed allowance that suits every local A-road journey. Adding the same number of minutes regardless of distance would create certainty that the national average cannot provide. Using mileage at least makes the starting allowance responsive to journey length, while leaving the final contingency as a personal planning decision.
Choose the extra margin
Use the longer of the existing journey estimate and the time implied by 24.6 mph as the working baseline. Where punctuality matters, add a further margin reflecting how much uncertainty you are prepared to accept. The official average does not quantify that cushion or guarantee an arrival time, so it is better treated as a check on an optimistic plan than as an exact timetable.
Before leaving, identify the local A-road mileage, calculate its 24.6 mph baseline and compare it with your schedule. Allow at least the difference if your plan is shorter, then add a personal margin if arriving late would be difficult.