Public transport accessibility level
PTAL is a simple, easily calculated approach that hinges on the distance from any point to the nearest public transport stop, and service frequency at those stops. The 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 PTAL calculation was originally developed by the London Borough of Hammersmith and Fulham, and was later adopted by Transport for London as the standard method for calculation of public transport access in London. It is not commonly used outside Greater London or the south east of England.
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).
The PTAL is used as a development planning tool in London, to determine both permitted parking standards and development densities. Large site developments (those the London boroughs refer to the Greater London Authority) must follow planning guidelines that allow more parking in areas with low PTALs (i.e., poor public transport) and vice versa—and that also relate the allowed density of development to PTAL (i.e., areas with better public transport may 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 isochrone maps – 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.
- Transport for Greater Manchester developed the Greater Manchester Accessibility Level (GMAL) method. This is similar to the PTAL calculation but uses "crow flies" distances to Service Access Points. It also includes an additional score for "Local Link" demand responsive transport services.
- London Datastore http://data.london.gov.uk/dataset/public-transport-accessibility-levels/resource/86bbffe1-8af1-49ba-ac9b-b3eacaf68137
- Greater Manchester Transportation Unit, http://www.gmtu.gov.uk/gmbusroute/GMAL%20Calculation%20Guide.pdf
- Parking standards in the London Plan, which are linked to PTALs
- Advances in public transport accessibility assessments for development control – a proposed methodology – a paper presented to a PTRC conference proposing an alternative accessibility modelling approach to public transport accessibility calculations
- – TfL PTAL Calculator with map to select point