Traffic and Travel: Data Definitions and Caveats

This section of the database provides information on the number of journeys people make, the distance they travel, the types of transport (modes) that they use and the purposes for which they travel. It also tabulates the total passenger kilometres travelled by people by different types of transport.

This travel results in the movement of vehicles, and the '/defn_traffic/traffic/index.html' related information lists the total movement of vehicles on public roads measured in vehicle kilometres, and its distribution by types of vehicle and classes of road.

Vehicle kilometres of traffic on a class of road or by a type of vehicle are usually estimated on the basis of vehicle counts at a number of points. Every so often, the model for estimating traffic from counts of vehicles is re-calibrated on the basis of more widespread counts. This can lead to a discontinuity in the trend of traffic from year to year, which can be as large as 5% overall or 10% on particular types of road.

The traffic each year relates to the public road network in place in that year. Thus growth over time is the product of any change in the network (kilometres) and the change in traffic flow on each link (vehicles per year).

Some countries, such as Russia, China, India and Brazil, have little or no data on passenger travel by car.

The total activity of traffic on the road network is measured in vehicle kilometres. The traffic each year relates to the public road network in place in that year. Thus growth over time is the product of any change in the network (km) and the change in traffic flow on each link (vehicles per year). In the USA, the volume of traffic is often referred to as 'Vehicle Miles Travelled' (VMT).

Vehicle kilometres of traffic on a class of road or by a type of vehicle are usually estimated on the basis of vehicle counts at a number of points. The method used in Britain is described in a Department for Transport paper How the National Road Traffic Estimates are made" (August 2007). At intervals, the estimation model is recalibrated on the basis of more widespread counts. This can lead to a discontinuity in the variation of the traffic flow from year to year. For example in 1999, the estimated traffic in Britain was revised, leading to a reduction of 4.6% on all roads and an increase of 10.2% on motorways. This indicates the potential uncertainty in the estimates of traffic on a complete road network.

The US National Transportation Statistics 2005 Appendix C provides the following abridged description of the estimation of traffic volumes: " Highway vehicle-miles of travel (vmt) are estimated using data from the Highway Performance Monitoring System (HPMS). Annual vmt by highway functional system is calculated as the product of the annual average daily traffic (AADT) along each highway section, the centerline length of each highway section, and the number of days in the year. Also, expansion factors are used for roadways that are sampled rather than continuously monitored. Vmt by vehicle type is estimated using vehicle share estimates supplied by states. The sampling procedures suggested are designed to produce traffic volume estimates with an average precision level of 80-percent confidence with a 10-percent allowable error at the state level. FHWA adjusts questionable data using a variety of standard techniques and professional judgement."

Time series for traffic will be affected by changes in the classification of roads. Thus the transfer of roads from national to local government that is taking place in many countries will lead to a reduction in vehicle kilometres on national roads and an increase on local or regional roads.

Definitions of types of road are given in 'Demographic and General Data'. Definitions of vehicle types are given in 'Vehicles'. However, in some countries, definitions of vehicle types for traffic are different to those for licensing. Thus in Britain, for traffic counts, 'Cars and taxis' include motorised invalid carriages and passenger vehicles of less than 3.5 tonnes gross weight and with 15 or fewer seats (for licensing, cars have 8 or fewer seats).

In the Transport Statistics database, 'All motor vehicles' include powered two-wheelers, unless these are specifically excluded. Motor cycles include scooters, mopeds and all motor cycle or scooter combinations.

Traffic counts generally classify goods vehicles by the number of axles and whether they are rigid or articulated (which usually include drawbar trailer combinations). In Britain, between 2001 and 2002, traffic counts of light vans and 2-axle heavy goods vehicles were revised to move some vehicles from HGVs to light vans, as they had been improperly classified.

In Britain this is classified as personal travel (travel for private purposes or for work or education, provided the main reason for the trip is for the traveller to reach the destination) and trip (the basic unit of travel defined as a one-way course of travel having a single main purpose. Outward and return halves of a return trip are treated as two separate trips).

In the USA this is classified as a person trip (a trip taken by an individual) and person-miles (an estimate of the aggregate distances travelled by all persons on a given trip based on the estimated transportation-network-miles travelled on that trip).

The basic data for most countries are passenger kilometres travelled by all modes and by the main modes. Divided by population, these give the average distance per person per year by mode. These estimates of distance travelled per person are supported by the results of national travel surveys, where these are available.

For modes such as pedal cycle and walk, the distance per person per year comes from travel surveys, and the national passenger kilometres obtained by multiplying by the population. Only a few countries have data on the distances people walk.

These measures of travel have limitations and uncertainties, which vary between countries and types of transport. In most countries car mileage, and hence passenger kilometres by car, is on public roads only. Obtaining passenger kilometres by car from estimates of car traffic requires the average number of passengers per car to be known. This is obtained in a variety of ways, and involves an uncertainty that must be several percent.

The US National Transportation Statistics 2005 Appendix C provides the following abridged description of the estimation of passenger car travel: "Passenger-miles of travel (pmt) are calculated by multiplying vmt estimates by vehicle loading (or occupancy) factors from various sources, such as the Nationwide Personal Transportation Survey. Thus, pmt data are subject to the same accuracy issues as vmt, along with uncertainties associated with estimating vehicle-loading factors."

In Britain, the distance walked is only recorded for walks alongside roads and in parks. The distance by pedal cycle is on public roads only.