Public Transport Accessibility Level
Encyclopedia
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, 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.
, 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.
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).
) 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.
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.
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
transport planning to assess the access level
Accessibility
Accessibility is a general term used to describe the degree to which a product, device, service, or environment is available to as many people as possible. Accessibility can be viewed as the "ability to access" and benefit from some system or entity...
of geographical areas to public transport.
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.
Background
The PTAL calculation was originally developed by the London Borough of Hammersmith and FulhamLondon Borough of Hammersmith and Fulham
The London Borough of Hammersmith and Fulham is a London borough in West London, and forms part of Inner London. Traversed by the east-west main roads of the A4 Great West Road and the A40 Westway, many international corporations have offices in the borough....
, and was later adopted by Transport for London
Transport for London
Transport for London is the local government body responsible for most aspects of the transport system in Greater London in England. Its role is to implement the transport strategy and to manage transport services across London...
as the standard method for calculation of public transport access in London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
. It is not commonly used outside Greater London
Greater London
Greater London is the top-level administrative division of England covering London. It was created in 1965 and spans the City of London, including Middle Temple and Inner Temple, and the 32 London boroughs. This territory is coterminate with the London Government Office Region and the London...
or the south east of England.
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 busBus
A bus is a road vehicle designed to carry passengers. Buses can have a capacity as high as 300 passengers. The most common type of bus is the single-decker bus, with larger loads carried by double-decker buses and articulated buses, and smaller loads carried by midibuses and minibuses; coaches are...
stops and rail
Rail transport
Rail transport is a means of conveyance of passengers and goods by way of wheeled vehicles running on rail tracks. In contrast to road transport, where vehicles merely run on a prepared surface, rail vehicles are also directionally guided by the tracks they run on...
stations (where rail can be taken to also include London Underground
London Underground
The London Underground is a rapid transit system serving a large part of Greater London and some parts of Buckinghamshire, Hertfordshire and Essex in England...
, DLR
Docklands Light Railway
The Docklands Light Railway is an automated light metro or light rail system opened on 31 August 1987 to serve the redeveloped Docklands area of London...
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
Piccadilly Line
The Piccadilly line is a line of the London Underground, coloured dark blue on the Tube map. It is the fifth busiest line on the Underground network judged by the number of passengers transported per year. It is mainly a deep-level line, running from the north to the west of London via Zone 1, with...
from Hammersmith
Hammersmith
Hammersmith is an urban centre in the London Borough of Hammersmith and Fulham in west London, England, in the United Kingdom, approximately five miles west of Charing Cross on the north bank of the River Thames...
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
Headway
Headway is a measurement of the distance/time between vehicles in a transit system. The precise definition varies depending on the application, but it is most commonly measured as the distance from the tip of one vehicle to the tip of the next one behind it, expressed as the time it will take for...
). 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).
Uses
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 AuthorityGreater London Authority
The Greater London Authority is the top-tier administrative body for Greater London, England. It consists of a directly elected executive Mayor of London, currently Boris Johnson, and an elected 25-member London Assembly with scrutiny powers...
) 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 isochrones - journey time contours - within which the number of workplaces, households or residents can be calculated using census
Census
A census is the procedure of systematically acquiring and recording information about the members of a given population. It is a regularly occurring and official count of a particular population. The term is used mostly in connection with national population and housing censuses; other common...
data. This method takes into account many more factors than PTAL, but is much more time-consuming and requires expensive software.
External links
- Map showing PTAL areas in London
- Details of the PTAL calculation (RTF format)
- 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
- http://webpid.elgin.gov.uk/ - TFL PTAL Page