Traffic / Cycle Counter Appendix – January 2015
1. Automatic Traffic/Cycle Counter information
Traffic levels vary with the season, weather and time of day. BHCC operate a number of traffic and cycle counters for its long term permanent monitoring work.
The counters are installed as cabinets on the road side which are connected to inductive loops buried in the road. Powered by a combination of batteries and solar panels, they work by detecting passing traffic overhead much like a metal detector. There are currently 12 active traffic counters and 13 active cycle counters around key points in the city. Each counter detects traffic crossing in both directions and are set up to classify vehicles by their type (e.g. lorry, van or car) and speed.
The information is gathered and stored on memory in the counter, and downloaded every month to a database held on the councils system.
Permanent data collection sites provide knowledge of seasonal and day of week trends. The summarization of this data can be applied to shorter temporary counts, to give an estimate of how the count may vary over the seasons.
They also give a measure of the varying levels of traffic flow in different areas of the city, and throughout the city as a whole. As part of the council’s efforts to increase more sustainable modes of transport, the counters provide valuable information on the usage of these different modes of transport.
2. Missing data procedures
Getting accurate data from the traffic counters is important as it helps BHCC understand travel patterns and allow the development of factors to convert more short term traffic counts. However due to vandalism and malfunction, it is not always possible to get a full data set; the counters often contain missing data. This is a standard problem with traffic and cycle counters, and for the usage requirements does not present serious flaws. BHCC has a set procedure for dealing with missing data that is clear and conservative with regards to yearly trends, outlined as follows.
2.1. Monthly figures (MADT)
All data from the counters is collated into monthly average daily traffic values. These figures represent the average flow of traffic a day for that month. These values take into account all days from that month and when a few days data capture is missing, the denominator is reduced by the number of missing days. Missing monthly data is patched with the equivalent month’s data from the previous year. In addition, data without any values for the previous year was patched with data from the subsequent year. This procedure does not make any assumptions about growth; it is conservative with regards to changes in volume from year to year, and takes into account the seasonal variation in traffic from month to month.
2.2. Annual figures (AADT)
When calculating annual figures (Annual Average Daily Traffic – AADT) monthly figures are used alongside denominators based on the months’ length in days.
Some (newer) counters do not have any data for a particular month (e.g. December). When this is the case, AADTs are calculated as if the month does not exist by only calculating the average daily traffic with days in the year which are represented. When data becomes available, these AADTs will necessarily be updated, but are provided to allow short term evaluation of any trend.
2.3. Rolling back to original data
The amount of missing data for a counter can be determined from the raw data files, in which the new value is not substituted but instead linked by a formula, allowing the reconstruction of the original data set.