National Traffic estimates: An overview

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Annual Average Daily Flow (AADF), or Annual Average Daily Traffic (AADT), refers to the average number of vehicles passing along a stretch of road in a day, taken over the course of a year. These values are often disaggregated by vehicle type, but usually exclude pedestrian traffic. 

Daily traffic counts can be used, alongside the length of the road, to calculate the number of miles (or kilometers) travelled by vehicles along the road each year. This gives a measure of vehicle exposure along the road, often referred to as (annual) traffic volume. It is often reported in millions of vehicle miles (MVM), or millions of vehicle kilometers (MVKM). 

Traffic Volume= (AADF)×(Road Length)×365.25Traffic Volume= AADF×(Road Length)×365.25

The leading source of AADF in Great Britain is the Department for Transport (DfT) Traffic Count Points. These provide traffic counts for every link on the motorway and A-road network, as well as a number of minor roads. An interactive map shows the locations of count point locations, and data can be downloaded back as far as 1993. The DfT traffic count data can be accessed here.

There are two main ways in which the DfT traffic counts are measured: manual counts and automatic counts. The Road Traffic Estimates Methodology Note contains information on both of these approaches. Click here to read the methodology note.

Manual traffic counts are conducted from 7am to 7pm on a sample of days between March and October by trained enumerators. Sample days are chosen due to weather and light considerations and exclude public holidays and school holidays. In total, around 8,000 manual counts are carried out each year. 

As it is not possible to cover every count point location every year, major roads are surveyed either annually, or on a cycle of every 2, 4 or 8 years, depending on traffic levels. On minor roads, a representative sample of roads are chosen each year for manual counts. Growth between years is then applied to previous manual traffic counts to estimate counts for the most recent year where manual counts are not available. 

The DfT own and operate Automatic Traffic Counters (ATCs) covering approximately 300 locations across Great Britain and can be used to measure the in-year variation in traffic across different road types and vehicle types. They consist of an Inductive Loop paired with a Piezoelectric Sensor to record information such as vehicle length and wheelbase, which can be used to classify vehicle types. In addition to these, approximately a further 10,000 ATCs operated by Highways England, 900 ATCs operated by Transport Scotland, and 300 ATCs operated by Transport for London are used to supplement the DfT’s automatic traffic counts. 

Other sources of traffic counts are available. In-vehicle telematics counts can be adjusted to provide an estimate of AADF across the entire road network. However, smaller sample sizes can impact the accuracy of these estimated counts on roads with lower traffic levels. Computer Vision technology applied to CCTV cameras can also produce automatic traffic counts as an alternative to Inductive Loops. 

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