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DfT >
AADFs > Estimates
How National Traffic Estimates Are Made
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1.
In
2006, motor vehicles travelled about 506 billion kilometres while pedal cycles travelled
4.6 billion kilometres along the public roads of
Great
Britain
. How do we know this? We know this from the traffic counts
conducted about many different types of roads and information on road lengths. However
it is not easy to convert count data to total traffic data.
2.
The
road network consists of about 50 thousand kilometres of motorways and class "A"
roads, with a further 338 thousand kilometres or so of minor roads. The road system
as a whole is thus much too extensive to allow the collection of comprehensive traffic
data for every part. Moreover, the density of traffic carried (and the mix of traffic
by vehicle type) varies enormously from place to place and from hour to hour. Flows
are less than 100 vehicles a day on many minor roads but are about 150
thousand a day on some motorway links.
Even within a road class, one site may easily carry ten times as much
traffic as another. Flow also varies by time of day, by day of week and by month
of year. Furthermore, there is variation within the variation - car traffic levels,
for example, may change relatively little over the seven days of the week, but goods
traffic is usually at far lower levels on Saturdays and Sundays than on other days.
These characteristics mean that estimating national traffic volumes requires a fairly
complicated sampling design, the collection of substantial volumes of data and complex
computational procedures. This note describes the new methodology used for the traffic
estimates from 1993 onwards.
3.
Estimation
of traffic levels uses information from both manual and automatic counts. They are
each described briefly below.
The manual counts
(previously referred to as the rotating census and the biennial counts)
4.
These
counts operate somewhat differently for major and minor roads. The major roads are
split into five road classes: motorways, trunk roads and principal roads with the
latter two divided into urban and rural roads. Urban roads are defined as those
within the boundaries of the Urban Area polygons for settlements of 10,000 population
or more, based on the 2001 Population Census. On the outskirts of urban areas, bypasses
are normally treated as rural even if part of the road may lie within the urban
area polygon. Conversely, roads between urban areas with short lengths outside the
polygons are normally treated as urban. Minor roads are divided into 6 classes:
B class, C class and U (unclassified) roads, each sub-divided into urban and rural.
5.
For
major roads (motorways and A-roads), the traffic on every link - normally a section
of road between consecutive junctions with other major roads - must be regularly
assessed. This is done by counting the traffic at a statistically random point on
links at regular intervals, between every year and every 8 years, depending on the
level of traffic and its variability. It is recognised that with the exception of
motorways, traffic levels will vary along the length of a link. However, the procedure
of counting at a statistically random point on each link can be expected to lead
to good estimates at national level although estimates on some individual links
may be less reliable.
6.
In
total, about 4,900 major road sites are scheduled to be counted in 2007.
In addition to traffic count data, information is collected about the characteristics
of each link, such as its length and the road class and road width at the place
of the count. At each chosen point, trained enumerators count vehicles of each of
eleven types (pedal cycles, two-wheeled motor vehicles, cars and taxis, buses and
coaches, light vans, and six separate categories of goods vehicle) for the 12 hours
from 7am to 7pm. These counts are all scheduled to take place on weekdays, but not
on or near to public holidays or school holidays. To minimise the effects of possible
seasonal factors, counting is confined to the so-called "neutral weeks".
These are namely most weeks in March, April, May, June, September and October.
7.
Some
major road links are unsafe to count or are too short to be worth counting in the
normal way. In these cases, traffic estimates are derived from the judicious use
of flow data on adjacent links. These are called derived links. Further, because
all links are now defined as ending at a local authority boundary, some links are
treated as dependent links. In these cases, it is assumed that the flow is the same
along all of the link. So, a count in one local authority can be used as a proxy
for the flow on the dependent link. In the calculation of 2006 annual road traffic
estimates, there were 15,609 normal links, 1,240 derived links and 1,041 dependent
links. Complete coverage of the minor road network is not attempted as it
is too extensive. It is not practicable to define the minor road network in terms
of individual links; even if all the links could be identified, their number would
be far too great to allow traffic data for each link to be collected. Minor road
traffic estimates are therefore made by grouping minor roads into one of the six
road classes. An attempt is then made
to measure the average flow on each of these road types by carrying out a number
of counts along them. A random sample of approximately 4,500 sites across GB is
visited each year. These same sites are counted each year.
Most of these counts are carried out in neutral weeks.
8.
The
manual counts have the advantage over automatic counts of complete coverage of major
road sites and moderately good coverage of minor roads. However, the data (hourly
by vehicle type), are very sparse since traffic is counted for only 12 hours on
each visit. Thus these counts give no information about traffic at night, at weekends,
over public holiday periods, and little about the seven non-neutral months. In calculating
national traffic estimates, therefore, use must be made of data from automatic counters.
The automatic
counts (previously referred to as the core census)
9.
The
automatic counters fill the gaps left by the manual counts. There are some 190 sites
in GB outside of
London
where traffic is monitored continuously using automatic sensors, which classify
the traffic into vehicle type. The numbers of vehicles of each type detected are
combined into hourly totals and stored on-site until it is downloaded during the
night to a computer in the DfT headquarters building. The automatic counting equipment
recognises 22 different types of vehicle; these are then combined to provide estimates
for the eleven vehicle types used by DfT.
10.
The automatic counters
do not give 100% accuracy. For example, the equipment cannot classify vehicles into
their different types when the traffic is moving very slowly (5mph or less). They
also have a tendency to malfunction, though the new sensors recently introduced,
are more reliable than the previous ones. The equipment cannot distinguish between
cars and car-based vans and can also have difficulty distinguishing between some
types of buses and coaches and goods vehicles having similar axle spacing and chassis
height. The equipment is also prone to failing to identify two-wheeled vehicle traffic,
both bicycles and motorcycles. Nevertheless, they do have
the big advantage over manual counts that they operate continuously and so can give
a complete picture of traffic at the points where they are sited.
11.
The automatic counters
in
London
are slightly different to those outside
London
. There are 54 automatic counters in
London
and they are "volumetric" classifiers that only distinguish between short (up to
5.2 metres) and long (greater than 5.2 metres) vehicles. They need 24-hour manual
counts every three months to provide estimates of the breakdown of traffic by vehicle
type in each hour of the day. These counters suffer from similar problems as those
outside
London, but are more reliable than the automatic
counters used outside
London
.
Annual Average
Daily Flows (AADFs)
12.
The data for all
manual counts done in neutral months are combined with information from
automatic counters on similar roads to provide an estimate of the AADF at that site.
This is normally done by multiplying the raw count data by factors derived from
the automatic counts in that same year. There are a large number of such expansion
factors since there are separate factors for each vehicle type, day of counting
and expansion factor group. Because these counts are
done in neutral weeks, the expansion factors used do not usually vary too much from
year to year, except when bad weather has restricted traffic during the winter months.
For cars, the factors are usually between 1.00 and 1.25 (except on motorways and
in London where the factors are higher) while for goods vehicles the factors vary
between 0.75 and 1.25 - lower because of the greater drop in goods vehicle traffic
at weekends.
13.
The automatic counters
provide a reasonable guide to changes in traffic over time. This information is
used in two ways. Firstly it is used to provide provisional quarterly estimates
of traffic, which are published on the sixth Thursday following the end of a quarter.
Secondly they are used to provide growth factors between consecutive years. These
growth factors are used for links not counted, or not counted satisfactorily in
the latest year. In these cases, the AADF for the previous year is multiplied by
the appropriate growth factor to give a reasonable estimate of the AADF for the
latest year.
Use of AADFs
in Calculation of Annual Traffic Estimates
14.
Different procedures
are used for major and minor roads in converting AADF data to traffic estimates.
The difference arises because the link concept cannot be applied to minor roads.
Major roads
15.
A major road link
of length 2km with an AADF of 50,000 has a traffic figure of 100,000 vehicle-kilometres
(2*50,000). This equates to 36.5 million vehicle kilometres a year. Because every
major road link is counted, in principle, total traffic on major roads can be obtained
by summing the traffic figures for every link.
16.
As mentioned in
paragraph 6, some links are not counted. In these cases, the traffic flows are derived
from adjacent links using suitable formulae (derived links) or using the flow of
the adjacent link as a proxy (dependent links).
Minor
roads
17.
In the base year
(currently 1999), for each minor road class in each local authority an AADF is estimated
based on a sample of traffic counts, including those projected forward from counts
done in earlier years. These AADFS are then multiplied by the total road length
for the relevant minor road category to give an estimate of traffic for that road
category.
18.
Traffic for the
latest year is then obtained by calculating changes in traffic flows, after taking
into account any changes in road length.
For the 2000 to 2003 estimates, the flows were derived from the automatic road counts. For the 2004, 2005 and 2006 estimates,
traffic flows from the manual counts were used.
This is plausible, since some of the newest will be quiet roads on housing
estates whilst others will be busy roads recently declassified from major road status.
Quarterly and
annual estimates of traffic
19.
As mentioned in
paragraph 12, the automatic counters are used to provide provisional quarterly estimates
throughout the year. A first estimate for the year is published in early February,
and this is largely based on automatic count data. A final estimate for the year
is normally published in early July and this is produced by putting together the
estimates for major and minor road traffic as detailed above.
Availability
of data
20.
Basic quarterly data are included
in the quarterly statistical electronic bulletins
Traffic in Great Britain, which can only be obtained from the DfT
web site. They can be downloaded from the correct PDF file which can be located
under
http://www.dft.gov.uk/pgr/statistics/datatablespublications/roadstraffic/traffic/qbtrafficgb/
21.
More detailed information is
available in the annual publication
Road Statistics 2006: Traffic, Speeds and Congestion.
This is available free by contacting
Road
Traffic
Statistics, but also available from
the DfT web site. It can be found at the following site:
http://www.dft.gov.uk/pgr/statistics/datatablespublications/roadstraffic/speedscongestion/roadstatstsc/roadstats06tsc
22.
Some users need
more detailed data for particular areas or particular roads. These can be supplied,
though there is a charge payable by external customers.
23.
The Department has undertaken a Quality Review of its road
traffic estimates, under National Statistics guidelines. The final report
has been published and is available, from the Office for National Statistics website,
at:
http://www.statistics.gov.uk/about/data/methodology/quality/reviews/transport.asp
SR 2
August 2007
Prior to 1993,
traffic was estimated for built-up and non built-up roads, with built-up roads defined
as those with a speed limit of 40mph or less
(irrespective of whether there are buildings or not) and non built-up roads as those
with a higher speed limit.. This definition
was originally introduced to identify roads in built-up areas. However, there has
been an increase in the adoption of speed limits of 40mph or less in rural areas
in recent years. As a result, the new classification of roads as urban or rural
(see above) has been adopted for traffic estimates from 1993.
These include: urban and rural 'B' roads,
'C' roads and unclassified roads
These include: Pedal cycle, two-wheeled motor
vehicle, car, light goods van, bus, rigid 2 axle lorry, rigid 3 axle lorry, rigid
4 or more axle lorry, 3 axle or 4 axle articulated, 5 axle articulated and 6 axle
or more articulated.
This deficiency underpins the need for the
extra sites for manual counting on minor roads in
the summer and winter months
Counts done in the summer and winter months
are not grossed-up: the sites are normally counted at the same time of the year
each year and so are compared directly with each other.
There are now 22 expansion factors groups.
These are based on type of area (from holiday area to
Central London), road category and, in some cases, traffic flow level.
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