Data Available in Northern Ireland: Road networks
Roads Service may be able to provide existing traffic count information. Automatic Traffic Counts (ATC) including vehicle classification and speed, Turning Movement Counts (TMC) and pedestrian counts. Where this is available there is a standard charge per site for providing ATC, TMC and pedestrian information. Roads Service divisional offices may be able to supply traffic signal information at a cost per site.
Ordnance Survey data
Ordnance Survey (OS) mapping for Northern Ireland provides an extensive range of digital vector information, produced from its Northern Ireland Survey and mapping archive. Information is held within a general-purpose digital topographic database available on Ordnance Survey website.
Other databases are available on the OSNI website including information on the road network, location of developments, public facilities, gazetteer of place names, local government districts, enumeration districts and other trip attractors.
Deriving walk and cycle times
Road networks can be used to derive walk and cycle access times. Distances between origin and destination pairs via the network are calculated and converted into time using an assumed average travel speed. It will generally be necessary to use the most detailed networks available but the following should be remembered:
- All roads in the network (including motorways and inappropriate roads) will be included in the calculation, but paths and other shortcuts will not.
- Euclidean (straight line) distances may be calculated with assumed average speeds, but that would not take into account the characteristics of the actual walking network, such as hills or pedestrian crossings.
Such data has to be viewed as inadequate for realistic walking and cycling catchment area calculation, unless specially adapted.
Deriving car drive times
A number of software packages are available (within GIS systems) which perform shortest path calculations through road networks and convert distances into times. In most cases the road network is divided into different road types and a database containing typical speeds by road type is used to derive link times. Routing algorithms are used to calculate a shortest path. Off-peak travel times and free flow traffic conditions are usually used in calculations. For the purposes of a Transport Assessment it might be necessary to represent a network for the AM or PM peak period. Although road speeds can be customised to reflect local conditions, the process to validate journey times in a network might be time-consuming.
An example of a software package to calculate road journey times is Autoroute. It is suitable to derive journey times at a national level. It includes major roads only and uses 10 categories of roads in its journey time calculation. Autoroute is used on an interrogative basis and only allows the user to look at one origin-destination pair at a time.