Method
The first stage in PTAL calculation is to calculate the walking distance from the site (known as the point of interest (POI)) to the nearest bus stops and rail stations (where rail can be taken to also include London Underground, DLR and trams). These stops and stations are known as service access points (SAPs)'. Only SAPs within a certain distance of the POI are included (640m for bus stops and 960m for rail stations, which correspond to a walking time of 8 minutes and 12 minutes respectively at the standard assumed walking speed of 80m/min).
The next stage is to determine the service level during the morning peak (defined as 0815-0915) for each route serving a SAP. Where service levels differ in each direction on a route, the highest frequency is taken. On railways, a route is generally defined as a service with a particular calling pattern – for example, services on the Piccadilly line from Hammersmith could be divided into two "routes": Cockfosters to Heathrow and Cockfosters to Uxbridge.
A total access time for each route is then calculated by adding together the walking time from the POI to the SAP and the average waiting time for services on the route (i.e. half the headway). This is converted to an equivalent doorstep frequency (EDF) by dividing 30 (minutes) by the total access time, which is intended to convert total access time to a "notional average waiting time, as though the route were available at the doorstep of the POI".
A weighting is applied to each route to simulate the enhanced reliability and attractiveness of a route with a higher frequency over other routes. For each mode (e.g. bus, Tube, DLR, tram, rail), the route with the highest frequency is given a weighting of 1.0, with all other routes in that mode weighted at 0.5.
Finally, the EDF and the weighting are multiplied to produce an accessibility index for each route, and the accessibility indices for all routes are summed to produce an overall accessibility index for the POI.
This accessibility index (AI) can then be converted to a PTAL grade (1–6) through a banding system (where AIs 0.00–5.00 are PTAL 1, 5.01–10.00 are PTAL 2, etc. up to PTAL 6 for scores of 25 and above).
Read more about this topic: Public Transport Accessibility Level
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