Theoretical PTAL values for combinations of local rail and bus services (summary)
This JRC report has modelled a combination of Accessibility Indices for a rail service and bus route density at development area – testing 4 and 6 trains per hour, bus service frequency and proximity of access to bus stops.
On its own, a 4 tph service would achieve an Accessibility Index (AI) of 2.7 4.7, and a 6 tph service an AI of 3.3 6.95. This is only just a PTAL 2 score (achieved at 5.01 and above), but not PTAL 3 (10.01+) nor PTAL 4 (15.01+). PTAL 3 and 4 are required, in order to be validated for higher housing densities under Greater London Authority (GLA) planning rules. So there is major reliance on the volume and proximity of a local bus network to drive the AI values up to PTAL levels 3 and 4.
Extensive modelling of a single bus route, through to a 4 bus route network, at varying levels of frequency, shows that the Accessibility Index is most sensitive to distance from a bus stop (=access time), followed by volume of routes and overall service frequencies. Modelling points to the best options being with effective bus stop catchments limited to 160 210 metres, and with a 3 or 4 bus route network in operation.
These principles can be applied to different areas within a development area, and some general judgments reached on how to optimise the bus routeing and stop locations. The solutions vary according to the sub sector served within the development. When applied to specific projects (not discussed in this paper), sensitivities will be identified, including the choices to be made in relation to existing or neighbouring bus routes, or the use of a new busonly corridor within the area.
The potential for cycling facilities to enhance AI values and help the PTAL score has also been reviewed. These have a small benefit (up to a quarter point of AI), of most use at development locations facing a long distance to reach the station.