PTAL stands for Public Transport Accessibility Level. It is a method sometimes used in United Kingdom transport planning to assess the access level of geographical areas to public transport.
PTAL is a simple and easily calculated approach which hinges on the distance from any given point to the nearest public transport stops and the frequency of the service from those stops. The final result is a grade from 1-6 (including sub-divisions 1a, 1b, 6a and 6b) where a PTAL of 1a indicates extremely poor access to the location by public transport, and a PTAL of 6b indicates excellent access by public transport.
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 doorstop 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).
The PTAL is used as a development planning tool in London to determine both permitted parking standards and development densities. Developments of large sites (i.e. those which the London boroughs must refer to the Greater London Authority are subject to planning guidelines which allow higher levels of parking in areas with low PTALs (i.e. poor public transport) and vice versa, and which also relate the allowed density of development to PTAL (i.e. areas better served by public transport can have higher density housing or offices).
TfL also have software to calculate PTALs across wide areas using GIS and timetable data, the typical result being a map with coloured bands relating to PTAL grades.
Advantages & disadvantages
Whilst PTAL is a simple calculation (easily performed by a spreadsheet) that offers an obvious indication of the density of public transport provision in an area, it suffers two key problems:
It does not take into account where services actually go to – for example, a bus that runs every ten minutes to the bottom of the road is considered better than a bus that runs every twelve minutes to the city centre.
The use of arbitrary cut-offs to exclude more distant service access points underestimates the ability to access locations just outside those cut-off distances. For example, a point 960m from King’s Cross could have a PTAL of 6, whilst a point 961m from the same station could have a PTAL of 1 or 2.
Accessibility modelling has been proposed as a solution to these problems. It uses GIS to calculate door-to-door travel times by public transport to a grid of points around the point of interest, resulting in a set of isochrones – journey time contours – within which the number of workplaces, households or residents can be calculated using census data. This method takes into account many more factors than PTAL, but is much more time-consuming and requires expensive software.