In order to estimate aggregate trends in traffic growth over time Transport Infrastructure Ireland (TII) has developed National Roads Traffic Indices. The indices have been calculated based on data from the TII Traffic Monitoring Units (TMU) combined with data from the TII National Transport Model. These aggregate trends complement the existing micro level data available at https://www.nratrafficdata.ie/c2/gmapbasic.asp?sgid=ZvyVmXU8jBt9PJE$c7UXt6.
An index is a statistical measure of changes in a representative group of individual data points over a time period. The Consumer Price Index (CPI) for example, is an index which tracks the variation in prices for a representative basket of consumer goods and services over time.
A well-constructed index is much easier to interpret relative to a situation where trends are being monitored from a series of individual data points.
That is to say, trends in prices (i.e. inflation) as expressed by a CPI index will be easier to interpret relative to a situation where inflation trends are being interpreted from a series of changes in individually priced goods and services.
Similarly, a traffic index across a region or road type will provide a consistent and interpretable measure of traffic growth trends of interest. This can be used in place of manually calculating a figure from a selection of individual traffic count sites which has the potential for inconsistency.
Broadly speaking, there are four main steps involved in calculating an index:
· identifying the relevant items (data points) to be included in the index;
· weighting the data points;
· monitoring the change in the data points over time; and
· creating an index from the aggregate weighted percentage change in the data points.
When created, an index number reflects the change in the representative group of individual data points compared with a standard or base value.
The base usually equals 100 and the index number is usually expressed as 100 times the ratio to the base value.
For example, if the average price of the representative basket of goods and services in 1990 is twice that recorded in 1980, the 1990 CPI index number would be 200 relative to 1980.
The TMU data provides continuous traffic count data at over 300 sites on the National Roads network.
These sites are grouped into regions or road types or other aggregations of interest to provide trends in traffic growth at each level.
The National Transport Model https://www.tii.ie/tii-library/strategic-planning/ is used to weight the influence of each traffic count site at a national level, regional level and at the other aggregations.
Traffic indices are provided on a quarterly basis with year on year percentage change also available over a full year of data. The indices are available for the following aggregations:
The 8 NUTS3 regions as defined by Eurostat and the Government of Ireland, as shown on this map.
Cross border sites on national roads between the Republic of Ireland and Northern Ireland. There is a total of 11 TMU sites located at or close to the border:
The three national roads types of motorway, national primary road and national secondary road:
The Transport European Transport Network (TEN-T) core and comprehensive routes in Ireland as defined by the European Commission:
The M50 and high traffic volume sections of the radial national roads connecting to the M50:.
Three categories of vehicles are provided as follows:
· All vehicles (Cars, Motorbikes, Light Goods Vehicles, Heavy Good Vehicles (Rigid and Artic) and Buses);
· Light Vehicles (Cars, Motorbikes and Light Goods Vehicles); and
· Heavy Vehicles (Heavy Good Vehicles (Rigid and Artic) and Buses).