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ASDT – Average Summer Daily Traffic
ASDT means Average Summer Daily Traffic. It is a seasonal traffic measure used where summer traffic is materially different from the average across the whole year.
There is not one universal statutory ASDT formula that applies to every transport assessment. The survey period and calculation method should therefore be defined in the traffic report or dataset using the figure. A summer average might, for example, be based on traffic observed over specified summer months or weeks.
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Why use a summer traffic average?
An annual average can hide strong seasonal peaks. Roads serving coastal resorts, visitor attractions, holiday accommodation, national parks and other leisure destinations can carry much more traffic during the summer than at quieter times of year.
In those circumstances, a transport consultant may use seasonal traffic data alongside annual and peak-hour flows to understand the conditions that matter to the development being assessed.
ASDT compared with AADF and AAWT
The Department for Transport’s national road traffic statistics use AADF – Annual Average Daily Flow, which is the average number of vehicles passing a point each day over a full year.
AAWT – Annual Average Weekday Traffic focuses on weekday traffic. ASDT instead focuses on a defined summer period. Which measure is most useful depends on the development and the traffic issue being tested.
How might ASDT matter in planning?
Seasonal traffic information can be relevant where a planning proposal may interact with unusually high summer demand. Depending on the site, it might inform:
- transport assessments and transport statements
- junction and access capacity
- visitor, tourism and leisure developments
- parking demand
- noise and air-quality work
- pedestrian and highway-safety assessments
Do not assume that an ASDT figure can simply replace the traffic data required by the highway authority. The appropriate survey period, peak periods and methodology should be agreed for the particular assessment.
ASDT Page Updated: 27th August 2026