CPB

Corporate Partnership
Board

London’s Accessibility
Indicators: Strengths,
Weaknesses, Challenges
Discussion Paper
Aliasgar Inayathusein
Transport Appraisal and Modelling,
Transport for London

Simon Cooper
Transport Appraisal and Modelling,
Transport for London

CPB

Corporate Partnership
Board

London’s Accessibility
Indicators: Strengths,
Weaknesses, Challenges
Discussion Paper
Aliasgar Inayathusein
Transport Appraisal and Modelling,
Transport for London

Simon Cooper
Transport Appraisal and Modelling,
Transport for London

The International Transport ForumThe International Transport Forum is an intergovernmental organisation with 59 member countries. It
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Cite this work as: Inayathusein, A. and S. Cooper (2018), “London’s Accessibility Indicators: Strengths,
Weaknesses and Challenges”, Discussion Paper, International Transport Forum, Paris.

LONDON’S ACCESSIBILITY INDICATORS | DISCUSSION PAPER | ITF

Table of contents
Introduction ................................................................................................................................................... 4
Existing connectivity measures ...................................................................................................................... 7
PTAL ............................................................................................................................................................ 8
Location-based catchment analysis ......................................................................................................... 14
London-wide catchment analysis ............................................................................................................ 19
ATOS: Access to opportunities and services............................................................................................ 23
WebCAT.................................................................................................................................................... 28
Measures under development ..................................................................................................................... 30
CYTAL ........................................................................................................................................................ 30
Methodological issues to be resolved ..................................................................................................... 31
Walking connectivity ................................................................................................................................ 32
Conclusions: Challenges and opportunities ................................................................................................. 35
Communicating technical concepts to a non-technical audience .......................................................... 35
Updating our datasets ............................................................................................................................. 35
Private modes/highway times ................................................................................................................. 35
Connectivity is not just about journey time ............................................................................................ 36
Formalised connectivity indicators vs ad hoc analysis ............................................................................ 36
Notes ............................................................................................................................................................ 37© OECD/ITF 2018

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Introduction
This paper provides an overview of Transport for London’s (TfL) suite of connectivity tools andmethodologies. Connectivity is the term used within Transport for London to describe measures which
may in other places be referred to as accessibility indicators. Collectively these indicators describe theextent to which users are able to access the transport system and travel freely between locations toaccess jobs and services. They measure the level of connectivity which the transport system provides for
citizens.
The first section of this discussion paper presents background material explaining London’s current andfuture challenges that highlight the importance of connectivity analysis. The next section provides an
overview of each of the existing connectivity tools explaining the purpose of each tool, calculation
methodology, strengths, limitations and example use cases. This is followed by a section that discusses
new measures currently under development. The final section offers discussion through a series ofquestions around unresolved challenges and areas for improvement.
London is a dynamic and dense city which has the challenge of accommodating a population of
8.7 million people and 26.7 million daily trips. This is expected to grow further by 2041 with an estimatedpopulation of 10.5 million and 32 million trips1 being made.
London is around 1 600 km2 in size with a dense public transport system serving much of the city. Innerand Central areas are well served by radial rail links, high frequency bus services as well as a significant
and increasing amount of high quality cycling infrastructure. The Central area and some parts of Inner
London are served by a cycle hire scheme. The public transport offer is more limited in Outer London but
still caters well for radial travel in to the centre through rail links and travel to and around town centres
via bus services. The Public Transport Accessibility Level map below highlights the differing public
transport provision throughout London with those at level 6b having the highest level of access to public
transport service and those at level 0 having the lowest.
The new Mayor’s Transport Strategy2 (MTS) currently published in draft, puts at its core reducing theneed to use cars in order to promote healthier and more sustainable travel choices. The key aim is tochange the way people travel so that by 2041, 80% of all Londoners’ trips will be made on foot, by cycle
or by public transport. This would be an increase of 16 percentage points from today’s 64% share and intrip terms, an increase of trips made by sustainable modes of around 50%. This ambitious target will be
met through promoting a “healthy streets” approach to policy making whereby health outcomes will beprioritised in planning London’s transport system. Further, the MTS looks to prioritise provision of a good
public transport experience whilst creating new homes and jobs.

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Figure 1. Greater London Public Transport Access Level Map

It is worth noting that London has also achieved many successes in the past few decades when it comesto sustainable travel. The mode share comparison chart given below in Figure 2 shows how London wentfrom having the lowest public transport mode share of the selected major European cities in 1995 tohaving the highest in 2012. The subsequent chart in Figure 3 highlights the principle reason for thischange: a strong and consistent growth in the use of all public transport modes in the last 15 years inexcess of population growth and a steady fall in car driver trips over the same period.
This shift has been achieved through a consistent effort made across all areas to promote sustainabletransport. Initiatives have included significant upgrades to bus, rail and London underground capacity,
introduction of road user charges with the Central London congestion charging zone and more recently
an ultra-low emissions zone (ULEZ) charge, as well as over GBP 1 bn investment in cycling. Alongsideinfrastructure investment, TfL champions sustainable planning policies for development in London andworks with businesses and schools to offer travel planning services promoting sustainable travel options.

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Figure 2. Public transport mode share across selected European cities

Figure 3. Growth in journey stages on selected modes, 2001-15

In order to plan the transport network as well as commercial and residential development in London,
connectivity analysis is key. This type of analysis, and the focus of this paper, enables TfL to understandwhere and to what extent sustainable transport options are available throughout London in accessing
jobs and other amenities. This is important as the availability of public transport alternatives is obviously
a clear driver of public transport mode share. Increased public transport use also correlates to higher useof active modes (walking and cycling) as these are often used as access and egress modes. This is shownin the chart below which shows the relationship between public transport access levels and active modeshares.
Overall, connectivity analysis plays an important role in informing London’s approach to planning
development and providing new infrastructure. The rest of this paper will explore this theme in greater
detail.

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Figure 4. Average trip rates in London by Public Transport Access Index

Existing connectivity measures
This section explains the connectivity measures and indicators that TfL has developed and maintains.
They can be categorised into four distinct groups based on their comparable methodologies:

access to the public transport network

access through the network - measuring location-based catchment statistics



access through the network - measuring London-wide catchment statistics



access through the network to specific opportunities and services (ATOS).

In this section we also include an overview of WebCAT our web-based connectivity assessment toolkit
which brings some of these measures together into a single interactive mapping application.

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PTAL
PurposePublic Transport Access Level (PTAL) is a well-established though relatively simple method of measuring
access to the public transport network in London. It is the only measure that has been formally accepted
and used on a statutory basis. For any location in London they combine walk access time to the public
transport with service availability at network access points (stations, bus stops etc.) within a given
catchment. A Public Transport Access Index (PTAI) is calculated from this data and allocated to a series oflevels where PTAL 0 represents the lowest and PTAL 6b represents the highest level of connectivity.
Figure 5. PTAL map of the Royal Borough of Greenwich

Calculation methodology

PTAL can be calculated for a single location or for a grid of points using the same methodology. They canhighlight those areas with low transport network density relative to others on a consistent basis. A grid
may cover a development site, an opportunity area, an individual borough or the whole of Greater
London (Figure 1). Mapping the results provides a simple and intuitive view of the density of London’spublic transport network that is understandable to a wide range of users. A sample map for the London
borough of Greenwich is given in Figure 5.
There are four main data components which are needed to create a PTAL dataset and are itemised inTable 1.

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Table 1. PTAL data components
DatasetDescription

Point(s) of interests(POIs)

These are the geographically defined locations where the calculation is performed. They can either
be individual points, a series of points representing key locations or a regular grid.

Service access points

These are also known as SAPs and represent the locations where you can access the public transport
network in London: bus stops, rail and tube station entrances, tram stops etc.

Walk network

This is used to calculate the walk distance/times between the points of interest and the network
access points. Walk networks could represent the existing network or a future walk network basedon a masterplan or other data source.

Valid service frequencydata

A database of selected service frequency data which itemises each service (and its frequency) thatstop at each SAP, based on their unique service/stopping pattern. The service data could represent
the current network or a modified network that includes proposed routes and schemes.

With access to the appropriate datasets, the PTAL calculation method can be broken down into a
series of stages which are itemised in Table 2.
Table 2. PTAL calculation processStage

Description

Calculate walk access time

From the point of interest(s) (POI) to nearby service access points (SAPs - bus stops, station
entrances etc.) via the walk network. Bus SAPs are only included in the calculation within 640 m
and rail SAPs within 960 m of the POI.
Distance is converted to time by assuming an average walk speed of 4.8 kmph. Thus 640 m isequivalent to an 8 minute walk and 960 m is equivalent to a 12 minute walk.

Link walk times with service
frequency data to create a“walk + frequency” output
file

In a database walk times are joined with route frequency data using the common SAP identifier.
Output the file including the following attributes:

Calculating PTAL

Import the “walk + frequency” output file into a spreadsheet to perform the following
processes:
 For each route calculate total access time combining: walk time + service wait time + a
mode-based reliability factor.
 Convert total access times to an “equivalent doorstop frequency (EDF)” to compare thebenefits offered by routes at different distances.
 For each grid point, remove all duplicate records (retaining the one with the highest EDF).
Note: a single POI may be able to access a single route at more than one location within the
maximum walk distance.
 Sum all EDFs with a weighting factor in favour of the most dominant route for each mode.
This gives the access index (AI) score of each service.
 Sum AIs by grid point to give the PTAI (Public Transport Access Index).

GridId, SAPId, RouteMode, RouteCode, vph

The PTAI is converted to the PTAL using the ranges specified in Table 3.

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Table 3. PTAL ranges

Usage

PTAL is TfL’s most widely used connectivity indicator partly explained by their use in the London Plan fora number of policy objectives. Although a relatively simple calculation they have the benefit of being
applicable at both the strategic or site level. Outlined below are the key areas where the measure is
used.

London Plan
TfL adopted and promoted PTAL in the early 2000s as a means to provide a consistent and agreed
methodology for measuring access to the public transport network across London. It became embedded
in strategic local planning processes in London, as a result of its inclusion in the London Plan.
The London Plan is the statutory spatial development strategy for the Greater London area and is writtenby the Mayor of London and published by the Greater London Authority (GLA). It sets out a fully
integrated economic, environmental, transport and social framework for the development of the capitalover the next 25 years. As a statutory document (under the terms of the Greater London Act [1999]), it
forms part of the development plan for Greater London. London boroughs’ local plans need to be ingeneral conformity with the London Plan, and its policies guide decisions on planning applications bycouncils and the Mayor. Consequently use of PTALs extends from GLA strategy to local boroughoperation. It should be noted that the plan is currently under-review and a new version will be issued inlate 2017. The current plan identifies three policy areas where PTALs are utilised:

prioritisation of the location of high-density employment generators in areas of high PTAL ratings
5 or above

relating housing densities to PTALs – where areas with good public transport provision (higher
PTAL) can support higher housing densities

as a means to restrict car parking provision in areas of good transport access or high PTALs.

Development area planning
PTALs are frequently used in strategic transportation studies as a means to show the impact of transport
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higher density housing. The measure has been used extensively to assess the level of public transport
provision in areas identified for development. In the London Plan, the largest of these are designated asopportunity areas. Generally, these are areas suitable for accommodating large-scale development toprovide a substantial amount of new employment and housing.
VNEB (Vauxhall, Nine Elms, Battersea) Opportunity Area is a regeneration area that spans 227 hectaresof central London on the south bank of the Thames, extending from Lambeth Bridge in the north, toChelsea Bridge in the south. The area’s comprehensive transport assessment included PTAL analysis
within the report. Figure 6 shows current PTALs, whilst Figure 7 shows PTALs in the future centred on
Battersea Power Station. Differences between the two maps are mainly due to the inclusion of:

the Northern Line extension to Battersea Power Station (extension of an existing tube line)

increased bus frequencies and stops across the area

improvements to the walk network in the area which also impact severance across the area.
Figure 6. Current PTAL map for the VNEB Opportunity Area

Figure 7 shows that in the vicinity of Battersea Power Station these enhancements will result in a rise to
PTAL 6a: a significant factor to achieving higher density development in this area.
Figure 7. Future PTAL map for the VNEB Opportunity Area

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General transport planning
In addition to the specific examples given above, PTAL is also a useful London-wide measure ofconnectively for a variety of studies:Prioritising a series of sites in terms of their proximity to the public transport network. Examples
have included rating NHS health service locations in terms of their PTAL rating: the most
preferred locations being ones with access to good public transport provision.

Providing a consistent connectivity measure for network planning: highlighting areas for further
investigation. Figure 8 for example, shows PTAL data displayed by mode only. Blue areas arethose with access to bus and rail services, whilst red areas are those with access to bus services
only. This map shows there are significant areas in inner London with access to bus services only
and may warrant further investigation in terms of new scheme development or scheme option
selection.

Combining with other data sources to support evidence based policy making decisions acrossTfL. Figure 9 gives one example of combining transport mode share ward level data (derived
from TfL’s Household Travel Demand Survey) with PTAL data. This shows how active modes
increase and car-based trip decrease as the PTAL “rises” across London.
Figure 8. Rail and bus connectivity in London

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Figure 9. Mode share and PTALs

Strengths and limitations
Key strengths of the PTAL methodology include:The output is intuitive and is relatively easy to interpret. The level of transport provision indifferent areas of London can be compared in a simple yet consistent manner.



The simplicity of the calculation method allows users to calculate values for individual locations
themselves using a spreadsheet.

They can be calculated at a micro or macro level. They are currently calculated for a 100 m gridof points across London. These results can also be translated into contours to produce a Londonwide and individual borough-wide PTAL maps – such as the example given above in Figure 5.

Acts as a good proxy for access to services and jobs in urban areas. With a well-connectedtransport network such as that found in London, those areas with a higher PTAL score will inmost cases be able to reach a wider range of service locations than those with a low score.

Provide a means of identifying the impact of changes to the walk and/or public transport
network at a local level with easy to interpret output.

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Limitations of the PTAL methodology include:

PTAL methodology only considers access to the public transport network. It does not take into
account how far you can travel through the network.

Since there are only nine PTAL categories, places with the same PTAL can still have significantly
different characteristics from each other in terms of their level of connectivity. Similarly a newscheme may raise the index value but still remains within the same category. In these cases theuse of the index values themselves may be a better approach.



PTAL is more limited as a measure of the benefit offered by large strategic projects such as newrail schemes as it only recognises improvements within a relatively local walk catchment. In
these cases a measure based on travel time analysis may be more suitable.



PTAL only includes walk time and service frequency explicitly in the calculation. Therefore any
schemes or interventions that improve station capacity, step-free access, reliability, or vehicle
capacity will have no impact on the level.

PTAL considers the frequency of services during the morning peak on a weekday. There are cases
where it could be more suitable to look at the level of connectivity at another time of the day or
during the weekend, for example around a shopping centre.

Location-based catchment analysis
PurposeThis type of analysis is conceptually simple yet very powerful. It uses travel time mapping and associatedcatchment data to provide additional assessment of connectivity, either by all public transport modes, ora specific mode, or by car. The concept of travel time mapping uses a series of time intervals to map
travel times from or to a selected location. Such analysis can be enhanced by associating the travel timebands with an appropriate count (sum) of population, employment or other services e.g. hospitals,
schools, retail locations) that are located within these time bands. This type of analysis has been madeavailable to a wider audience via our interactive web-based connectivity assessment toolkit WebCAT3
which is discussed later in this section.
It should be noted that TfL has not formalised an actual indicator using location-based catchment
analysis, rather it has focused on providing a suite of measures that can be adapted to fit the individual
users’ requirements.
Calculation methodology

Travel time data is derived from our strategic transportation model called Railplan. This model covers
London and the south-east, and assesses the likely routes and service choices of public transport users,
and the resulting flows, journey times and levels of crowding in and around London.
The Railplan model includes a detailed representation of the principal public transport services available
in London including: National Rail, London Overground, London Underground, Docklands Light Railway,
Tramlink and London Buses. In addition there is a detailed walk network to represent access to the public
transport system as well as transfers between services through station interchange links.

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The model includes public transport network information for the whole of London and its surroundingarea. Specifically this includes all buses services within the M25 cordon as well as those that intersectand extend beyond the GLA border. All rail services are included within London as well as those thatprovide connections between London and the rest of the country.
Railplan divides London into 3 288 zones for which we can extract travel time matrices giving journey
times for all combinations of origins/destinations. For this analysis we use non-generalised time torepresent “real” journey time via the shortest route and exclude penalties and weightings that are
utilised in the full generalised journey time (GJT) process.
Journey times include all elements of travel:
walk time between the zone centroid and the transport network

wait time for the public transport service

in-vehicle time - time of the public transport modeinterchange time.

Railplan represents average travel conditions in three time periods during a school term time weekday:

AM Peak (07:00-10:00)

Inter Peak (10:00-16:00)



PM Peak (16:00-19:00).

Railplan can also provide travel time data for different forecast years. The base year represents thecurrent state of the transport system (2011). Whilst future year transport networks have also beendeveloped for the principal modelling periods 2021, 2031 and 2041. For example, 2021 includes the
Elizabeth Line (Crossrail) and other service enhancements.
Usage

Location-based catchment analysis outputs are used widely throughout TfL and its stakeholder
organisations as a way to demonstrate transport characteristics in a transparent way as well as a means
to support evidence-based policy decisions.
Key uses include:

Scheme optioneering – quantifying the additional number of jobs or people that can be reachedwith the construction of a new transport scheme using different station locations and networkalignments.



Identifying the catchment characteristics of a specific location – for example the number of jobs
that can be reached from a residential site or the number of people who can access a newcommercial site. The later could consider the total population that reside within 30 minutes to a
shopping centre or the working age population that reside within a 45-minute commute to a
new office development.

Site selection and prioritisation based on the catchment characteristics. For example comparing
the number of potential employees who could access one site compared to another.

Below we provide some examples of how this type of analysis has been used in practice.

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Figures 10 and 11 give an example of how such analysis can be used to support the benefits ofinvestment in new infrastructure. These screen–shots are taken from TfL’s WebCAT tool anddemonstrate the impact on transport connectivity that the Elizabeth Line will have on Abbey Wood. The
map shows how travel times are reduced westwards along the Elizabeth Line’s alignment with most ofcentral London, accessible within 60-minutes travel time by 2021. This is underlined by the cumulative
bar charts showing that currently 0.72 million jobs can be reached within 60-minutes travel time, but thiswill increase to 2.4 million by 2021.
Figure 10. Travel times from Abbey Wood using the current network with the associated cumulative bar
chart showing 2011 jobs reachable within each time band

Figure 11. Travel times from Abbey Wood using the 2021 network with the associated cumulative bar
chart showing 2021 jobs reachable within each time band

Figure 12 demonstrates how the results for two scenarios and one location have been displayed on asingle map. The blue zone indicates travel time within 60 minutes to North Greenwich before
construction of the Jubilee Line extension to Stratford. Whilst the red zone indicates the extent of the
60-minute catchment zone after its construction. In this example North Greenwich previously had
limited connectivity so the impact of the Jubilee Line is greater than in areas where there is already ahigh level of connectivity (for example central London). As with previous examples travel time mapping
could also be quantified in terms of the number of people or jobs within each catchment area.

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Figure 12. 60-minute catchments to North Greenwich – with and without the Jubilee Line extension

Figure 13 illustrates how travel time data can be displayed for multiple destinations. These could be, forexample, a series of hospitals, schools, libraries or other service locations. The map shows the minimum
travel time to any selected location, the underlying assumption being that people will travel to theirnearest site.
Figure 13. Combined catchment areas for a series of service locations

Figure 14 shows travel times to the future distribution of services where the central location is closed
and travel times are based on the remaining locations.

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Figure 14. Revised catchment areas for a series of service locations

Perhaps the most interesting output is Figure 15, plotting the increase in travel times caused by theclosure of the central facility. As expected, the most significant increase in travel time (>5 minutes)
occurs in the vicinity of the facility, although other areas are affected due to the configuration of thetransport network. These outputs could also be arranged as a series of catchment statistics tabulating
the impact on the resident population of the service reconfiguration. TfL has worked in collaboration
with various service providers in London, including the NHS, providing travel time data and expertise toundertake this type of connectivity analysis.
Figure 15. Difference in travel times between the two scenarios (Figures 13 and 14)

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Strengths and limitations

Key strengths of the methodology include:The output is easy to interpret and present to both professional planners and the general public.

Provides an effective means of comparing the impact that transport interventions are likely tohave on key indicators of economic growth (e.g. access to employment) or access to services.



Individual or groups of modes can be analysed. For example travel times for the full network
could be compared with those for the step-free network to highlight the differences inconstraints experienced by step-free network users.

Key limitations of the methodology include:

These model outputs are not well suited for looking at local changes to the network such as new
local bus routes or improvements to walk networks. Travel times are based on the transportmodel zone distribution which typically divides each borough in London into about 100 zones.
The outputs therefore are best suited for use at a strategic level considering the impact of major
schemes and packages of schemes across London.

Travel time analysis by its definition looks at travel times alone. When assessing the impact of
new schemes, journey time reduction may not be the principal benefit. Other factors may be ofgreater significance: increasing network capacity, crowding reduction, enhanced passenger
experience etc. For example: journey complexity involving numerous interchanges could be ofgreater significance for some user groups than achieving the fastest journey time.

London-wide catchment analysis
PurposeRather than simply generating travel time plots for individual locations, London-wide catchment analysis
aggregates the catchment results for each zone/location in London (for example the number of jobs
reachable within 45 minutes from each zone) and maps the calculated catchment statistic rather than
the actual travel time values.
This method has been used most frequently in the London context with population and employment
data to provide a strategic view of changes in connectivity across the city.
Calculation methodology

The data used in this analysis is derived from Railplan standard origin/destination non-generalised (real)
travel time matrices. Travel times from and to every transport zone (over 16 million records) are stored
in a matrix format and are imported into Excel with appropriate zone based population or employment
statistics added to the matrix rows and columns. With this format population or jobs reachable withinsay 45 minutes for each zone can be calculated and the results plotted using a GIS.
Although the calculation method is simple there are a number of factors that should be considered when
preparing the outputs:“To” or “from” the zone. Employment catchments will be of most interest for residential
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45-minute travel time. Outputs will be based on travel time data “from” each zone. Population
catchments will be of most interest where there are commercial/retail land uses, i.e. quantifying
the number of people who can access each zone within 45-minute travel time. Outputs will bebased on travel time “to” each zone.


Selecting the most appropriate catchment time. 45 minutes has become the default TfL value
when displaying the number of jobs reachable from each zone. Recent LTDS survey analysis
suggests that 50 minutes is a more appropriate interval accommodating for longer commuting
times.

Usage

The examples below demonstrate how this methodology can be applied as a useful indicator combining
catchment variables with transport travel time datasets.

As a strategic indicator
Figures 16 and 17 show standard London-wide catchment analysis outputs produced by TfL using
workplace statistics. These maps quantify for each zone in London the number of workplaces that can bereached within 45 minutes by public transport. Although the maps focus on London they include zones
reachable beyond the London boundary. Such outputs provide a strategic indicator of connectivity in
London and are used in the current Mayor’s Transport Strategy document for this purpose. Comparing
Figure 16 with Figure 17 shows improvements in the transport network as well as general growth in
employment between 2015 and 2041.
Figure 16. Number of 2015 jobs reachable within 45-minute travel time using the full 2015 transport
network from each zone in London

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Figure 17. Number of 2041 jobs reachable within 45-minute travel time using the full 2041 MTS transport
network from each zone in London

As a measure of access to services
London-wide catchment analysis can also be used as a method to quantify the number of services
reachable within a given time period, replicating, to a certain extent the ATOS methodology which is
discussed later. Rather than calculate travel times to specific locations this method quantities the total
number of services that can be reached within a given time period. The example in Figure 18 is based on
two scenarios (with and without Crossrail 2) and displays how many more further education colleges arereachable within 45-minute travel times of each zone when Cross rail 2 is operational.
Figure 18. Change in access to further education colleges in London after the introduction of Crossrail 2© OECD/ITF 2018

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Measuring step-free accessTravel time matrices based on standard travel times and step-free travel times for the same scenario canalso be used with this method. TfL is committed to achieving an inclusive, accessible and affordable
transport network that benefits all Londoners. Considerable efforts have been made to reduce thejourney time differential between step-free and non-step-free journeys but this difference is still
considerable and is reflected in the fact that disabled people travel less often, making 1.6 trips perperson per day compared to 2.4 for those without a disability. The step-free network is a sub-set of thefull public transport network, excluding those station walk links which involve negotiating any series ofsteps. This could make some stations inaccessible or reduce the number of interchange options resulting
in longer journey times. A comparison of Figures 19 and 20 shows the impact on connectivity for users of
the step-free network in terms of the reduced number of further education colleges reachable within
45-minute travel time.
Figure 19. Connectivity to further education colleges using the current full public transport network

Figure 20. Connectivity to further education colleges using the current step-free network

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Strengths and limitations

Key strengths of the methodology include:

It provides useful indicator to demonstrate change in network connectivity across London at a
strategic level rather than focusing on specific locations.

This method is flexible and can be applied to a wide range of travel time datasets and catchment
statistics.



Displaying the number of service locations within a given time frame reduces the significance ofproximity to a specific (nearest) service.

Key limitations of the methodology include:

Use of outputs from strategic models limits the granularity of the data to the zone sizes used in
the model. Whilst the zoning system may provide sufficient granularity at a strategic-level, at theborough or local study level this may be too coarse.



Developing thresholds for what represents “good” or “bad” connectivity is difficult andunresolved.

This method provides a relative measure of connectivity. The absolute numbers could actually bemeaningless: for example is having access to 1 million jobs worse than having access to 2 million
jobs.

ATOS: Access to opportunities and services
PurposeAs already discussed, a significant limitation of the PTAL methodology is that it only considers access tothe public transport network and does not take into account how far you can travel nor theopportunities and services that can be reached.
In planning terms an agreed catchment-based connectivity indicator that takes into account London’s
dense urban environment, could support decisions on the most appropriate location for new facilities aswell as public transport services. Such a measure could help support the Mayor in setting regional
priorities, as well as improve collaboration between transport providers, such as Transport for London(TfL), with service providers, such as the NHS, in London.
For these reasons TfL developed a London-specific connectivity measure of access to opportunities andservices (ATOS) as a response to, and building on the UK’s Department for Transport (DfT) 2007 initiative
to publish core national accessibility indicators. Local authorities outside London were also required by
the DfT to set a target for at least one local accessibility indicator as part of their Local Transport Plan.
The indicators looked at the proportion of the resident population that can access a service within acertain time, for example: the percentage of 16-74 year olds within 20 minutes of an employment
centre. In the case of London, the local authorities (boroughs) were not obligated to report onaccessibility as Transport for London was and is accountable for ensuring good quality access. In addition,
most of the indicators set by the DfT would be met within the Greater London area.

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Although this measure was based on the DfT indicators, they were adapted to accommodate forLondon’s characteristics. For example ATOS allows for a degree of choice as to which services users arelikely to access.
Calculation methodology

Calculating ATOS involves a number of processes which are explained below.

Defining the origin points
ATOS requires a grid of points at an appropriate level of detail on which to base its calculations. ATOS
zones combine 2011 Census Lower Super Output Areas (LSOAs) with Census Output Areas (COAs), where
the former were felt to be too large, thereby giving greater granularity. This resulted in a total of 5 231
ATOS zones across London. Travel times are measured from each zone’s population weighted centroid toapproximate for where most people live within each zone.

Define the service/destination points
For the ATOS methodology we agreed a “basket” of key opportunities and services that were appropriate
to London:

Work: calculating average journey time to the nearest 10 000 low-qualified and high-qualifiedjobs



Education: calculating average journey time to the nearest three primary schools, secondary
schools and further education colleges

Health: calculating average journey time to the nearest three general practice/doctors surgeries

Quality food shopping: calculating journey time to the nearest town centre or supermarket

Open spaces: calculating walking time to the nearest publicly accessible open space

Calculating travel times
For each origin point we calculate the walk time/distance to the 10 nearest destinations for each service
set using a GIS network routing algorithm with a detailed London-wide walk network. For thoseorigin/destination pairs that were beyond agreed walking distances (more than 10 minute walk [800 m])
we calculated the public transport travel times. Like our London-wide catchment analysis methods,
travel time for ATOS is derived from the Railplan model. The 3 288 zones used in London are ideal for a
strategic catchment-based measure but in the case of ATOS more detailed analysis is required based on
point-to-point travel times. To accommodate for this requirement we use a variant of Railplan called
CAPITAL. CAPITAL strips away the standard model zones and allows the user to extract Railplan travel
times defined by their own origins and destinations. CAPITAL links Railplan with a GIS to consider thecomplete journey including walk time from the origin/destination to the PT network, service wait time,
in-vehicle time, and interchange time. A typical CAPITAL run for ATOS would therefore calculate travel
times from the LSOA centroid to the selected service locations.

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Calculating the ATOS scores
The public transport travel times calculated in CAPITAL are combined with the complete walk based
results in a spreadsheet. Any journey of less than 10 minutes is assumed to be a walk trip and for thoselonger than 10 minutes is assumed to be undertaken by public transport.
For the following services: GP surgeries, primary schools, secondary schools and further education, theaverage of the three fastest journey times is selected from a possible ten locations. Access to foodshopping is based on the travel time to the nearest store or town centre (assuming that an individual will
perform their main shop at one location and there are no capacity issues at the destination) whilst
journey time to open space is derived from the walking time to the nearest location only.
This gives us a list of average or minimum travel times from each zone and by each service type. Service
specific ATOS scores from A to E can then be derived using ranges based on the criteria given in Table 4.
Table 4. ATOS ranges
A

0 to (Mean – stdev)

B(Mean – stdev) to Mean

C

Mean to (Mean+stdev)

D(Mean+stdev) to (Mean+2 stdev)

E> (Mean+2 stdev)

Thus level A has the best or quickest level of accessibility to services and level E the worst or slowest. It
should be noted that the ranges derived for each service will differ according to the density of locations
across London. ATOS Level A for GP surgeries ranges from 0 to 7 minutes whilst ATOS Level A for Further
Education Colleges ranges from 0 to 22 minutes.
A composite score, combining individual service scores was created but resulted in an index that wasdifficult to interpret. This was achieved by combining the ranks of accessibility for each zone. The
composite score was computed by totalling the ranked average times by zone for each service and thensplitting the total ranks into five groups (A to E), where “A” was the 20% highest values and “E” the 20%
lowest values.
Usage
ATOS results can be either plotted using GIS software or analysed in a spreadsheet. Figure 21 shows the
ATOS scores based on average travel time to the three nearest secondary schools for each zone bywalking only or by walking and public transport modes. In this example category A corresponds to an
average travel time of up to nine minutes, whilst category E corresponds to an average travel time ofover 23 minutes. TfL has produced similar plots for all the services groups given above.

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Figure 21. Access to the three nearest secondary schools by public transport and/or walking

An important aspect of this work was to combine PTALs with ATOS to give a fuller picture of connectivity
across London. Figure 22 shows there is a strong correlation between access to services and access tothe public transport network. However, importantly, poor access to the public transport (e.g. PTAL 2 or
3) does not necessarily mean poor access to services.
The size of each blue circle represents the number of areas with a given PTAL/ATOS combination.
Generally access to services improves with access to public transport, although the correlation becomes
more diffused as access to services increases. So where there is good access to most services, public
transport provision becomes less relevant.
Whilst some areas have poor access to services, despite having good access to the public transport
network (Group A), a significant number of areas show good access to services despite poor access tothe public transport network (Group B). This suggests services in these areas can be accessed by walking.
TfL will investigate this approach further for assessing walking connectivity and the methodology is
discussed further in the following section.

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Figure 22. A comparison of accessibility when measured using PTALs and ATOS

Strengths and limitations

Strengths of the ATOS methodology include:Provides a transparent and understandable methodology for assessing connectivity to a basket
of services across London on a consistent basis.

Complements PTALs and can provide a more robust picture of connectivity

Helps identification of areas where many services can be accessed by walking or cycling. The
adoption of London’s new Mayor’s Transport Strategy and the move towards prioritisingsustainable forms of transport is giving greater emphasis to this type of analysis.

Limitations of the ATOS methodology include:Measuring access to specific services adds an additional level of complexity to these indicators.
ATOS includes travel times to the nearest three locations but does not consider service specificissues such as quality, capacity, public/private provision etc. Many people would consider theseissues more important than proximity to a service and would be willing to travel as far asnecessary to reach their desired location.

The original analysis used time bands based on the mean and standard deviation of each service
group. Thus each service group scores’ would have a different definition dependent on thedensity of locations available. Different ranges made it difficult to combine individual service
scores together in to one meaningful indicator. To ensure consistency and allow for time series
analysis we have since used fixed time intervals.

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High service density in London means that many facilities are available locally and can bereached by walking, cycling or by bus. In these cases the impact of the wider transport network(rail, underground etc.) would be less significant. This should be considered if a measure such as
ATOS is used to assess the impact of future strategic transport schemes.

Combined, these limitations have resulted in TfL not progressing development of the ATOS indicator inits current form. The preference is to undertake ad hoc location-based catchment analysis where theorigins and service destinations are more fully understood and defined.

WebCAT
PurposeThe most recent innovation TfL has made to its connectivity planning toolkit is the development andrelease of an interactive mapping tool: WebCAT. TfL’s Web-based Connectivity Assessment Toolkit isavailable on the organisation’s public web-site at tfl.gov.uk/webcat. A short promotional video
highlighting its key features and uses is also available at Youtube WebCAT video. WebCAT has been
designed to make our suite of connectivity tools, data and analysis more easily available to a wide
audience, primarily professional planners, but its simple and intuitive interface is also designed to be ofuse to the general public.
The London Plan obliged TfL to provide Public Transport Access Levels (PTAL) information to boroughsand developers as part of the policy to ensure sustainable residential housing densities across London. In
response TfL commissioned the development of an interactive web-site (WebPTALs) that calculated
PTALs for any user-selected location in London. The contract for WebPTALs ceased in 2015 offering theopportunity to develop a more ambitious web-site that included a wider variety of connectivity tools anddata. This formed the rationale for the development of WebCAT.
WebCAT offers two principal ways of measuring transport connectivity: PTAL and location-based travel
time mapping/catchment analysis. It is driven by an interactive mapping interface where the user is
expected to search or click on the map to return connectivity information (grid-based PTALs or travel
time plots) for their chosen location.
WebCAT core functionality and data includes:28

PTAL: London-wide isochrones or locally as 100-metre grid cells

Interactive travel time mapping (TIM): offering a range of travel time variables



Cumulative bar charts: population, jobs or services reachable within the travel time intervals
displayed on the mapComparison tool: compare travel time outputs for different travel time variables



Variable time bands



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Figure 23. WebCAT default screen screen-shot

WebCAT functionality gives access to a wide range of travel time and catchment datasets:
 PTAL calculated for the base/current year + 2021 and 2031
 Travel time mapping offers the following options:-scenario/year current, 2021 and 2031-

mode: all public transport modes, bus only, cycle only, step-free-time period: morning peak, inter-peak, evening peak-direction: to or from the chosen location

Catchment datasets include the following: total population 2011, 2021 and 2031
 households 2011, 2021 and 2031
 working age population 2011, 2021 and 2031
 economically active 2011, 2021 and 2031
 pensioners 2011, 2021 and 2031
 jobs/workplaces 2011, 2021 and 2031
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town centres: metropolitan, major, district-

educational: primary schools, secondary schools, further education-

health: GP surgeries, emergency (A&E) departments, pharmacies

Through WebCAT, TfL has endeavoured to offer relatively simple functionality that is useful to bothprofessional planners as well as the general public. The comprehensive list of WebCAT datasets
demonstrates the power and flexibility of the application in terms of the user being able to tailor analysis
to fit their individual requirements.

Measures under development
As stated in the introduction, the new Mayors Transport Strategy (MTS) places a significant emphasis on
shifting the travel mode split from car towards more sustainable modes of transport: public transport,
cycling and walking. This will also be reflected in our suite connectivity measures where we will need toinclude cycling and walking as equally valid modes of transport.
The MTS places an emphasis on “good growth” stating that this should ensure “that people living in new
housing in central, inner and outer London have options other than to drive to the shops, to school or towork. It means offering people across London – existing residents and new ones – the benefits ofwalking, cycling and public transport use that has been available in some parts of London for years.”
Walking and cycling connectivity will contribute towards the evidence base to ensure that “as London
grows, a greater proportion of people will live in locations that are well connected to employment andother opportunities by walking, cycling or using public transport.”
One example of this approach is already provided in WebCAT by the inclusion of cycling travel times
alongside other modes. The user can extract catchments statistics on a similar basis to those generated
for the full public transport network. WebCAT can also demonstrate that cycling is a realistic alternative
to other transport modes by comparing cycle travel times with public transport travel times.
This section now discusses additional new connectivity measures under development that accommodate
cycling and walking.

CYTAL
PurposeCycle Access Level (CYTAL) has been proposed as a means to encourage more sustainable residential
housing densities in London, specifically in areas with PTALs 0 to 2. CYTAL extends the PTAL catchment
area by including cycling as an access mode in addition to walking.
Using the standard PTAL methodology PTAL locations 0 to 2 will in most cases be beyond the maximum
walk distance (960 m – 12 minutes) to rail or underground stations. Rather than extend the walk30

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distance, the purpose of CYTAL is to substitute the walk access mode with the cycle mode. Faster accesstime by bicycle means a maximum cycle time of 12 minutes could translate to a maximum cycle distanceof 2.6 km at an average speed of 13 km/h. This would expand the access catchment area and raise thepotential for including more stations and services in the PTAL/CYTAL rating.
Calculation methodology
Figure 24 demonstrates how the CYTAL methodology could be applied in residential areas of London inorder to raise PTAL and support more sustainable residential densities. PTAL and CYTAL are calculatedusing a 100 m grid of points across the study area in Ealing, an outer London suburban borough. The
standard PTAL map shows that there are many residential areas with low PTAL scores mainly due tolimited access to bus services only. By inclusion of cycling as an access mode, connectivity levels rise
across the study area. The highlighted central area rises to PTAL 6 as a result of the addition of the three
local stations (A, B and C) in the calculation.
Figure 24. PTAL (left) and CYTAL (right) outputs for a typical suburban area in London (Ealing)

Methodological issues to be resolved
The concept of CYTAL is relatively simple but the methodology raises a number of issues andassumptions that will need refining during the development of CYTAL.
Maximum cycle access distance. For the purposes of this example we have assumed a maximum
cycle distance of 2 600 m and an average cycle speed of 13 km/h. We intend to undertake
further analysis of survey data from the London Travel Demand Survey (LTDS) to establish themost appropriate maximum distance for station access. It is envisaged that the maximum
distance will be relatively short because in this context cycling is only being considered as anaccess mode. A longer journey time where the final destination could be reached directly would
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should be defined. In circumstances where the station is within say 50 m of the origin point it isunlikely that an individual would using a bicycle and would instead walk. In the example above
we used 250 m as the maximum walk distance to stations but this assumption is open to review.


Accommodating for inner and outer London. It is envisaged that this measure will be most usefulin outer London residential areas with low PTAL scores where access by bicycle to a local station
could be a real advantage. In inner London the concentration of opportunities and services in a
relatively small area means that the most convenient mode of transport could be bicycle without
the need to access a station to reach the final destination. Furthermore in inner London there
are more locations with PTAL 6a or above – the highest levels. Applying this methodology would
not therefore result in a significant increase in the PTAL/CYTAL. Furthermore good existing
walking access to public transport will mean that cycling is more likely to be used as a main mode
rather than an access mode. As part of on-going analysis we will investigate using LTDS data toestablish the difference between access distance to stations in inner and outer London andincorporate these assumptions into the final methodology.



Access to bus services. CYTAL does not include access to bus services by bicycle. It is assumed
that bus services would be accessed by walking only.



Cycle infrastructure. The main purpose of this method is to highlight areas where the existing
PTAL is low but by including cycling as an access mode, the PTAL could potentially be raised. It
should be noted that the measure does not take into account the availability of cycle
infrastructure: the provision of cycle parking at either the origin or destination or the suitabilityof the cycle network for undertaking these trips. Furthermore no factor is included toaccommodate for the time it takes to park the bicycle.

Potential usage

We envisage that CYTAL is a methodology that would be used on an ad-hoc basis, in specific cases whereresidential property developers and planners could see the potential for cycling friendly development
and where public transport connectivity is possibly limited. As a strategic measure, it is expected thatCYTAL would highlight areas that could support such developments. It would form part of a suite ofevidence and resources to help inform discussions between planners, developers and otherstakeholders.

Walking connectivity
PurposeNew walking connectivity measures will be based on work already undertaken for the ATOS measurediscussed previously and will be particularly relevant for residential areas. Walking connectivity
indicators will aim to show how many, as well as the range of opportunities and services, that can bereached by walking for any chosen location in London.

Calculation methodology
Using a detailed walk network (based on Ordnance Survey ITN + Urban Path links) with a GIS network
algorithm we can generate shortest path routes between given origins and destinations, applying an
average walk speed to distance to give a walk time. Origin points could be defined as a regular grid of
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points at 100 m intervals and destination points a series of service locations. One example of thisapproach are the PTAL calculations in WebCAT where we calculate walk times from each grid point tonearby transport network access points within specified maximum walk distances. Further outputs at a
detailed granular level could be based on walk time to a range of service locations including: primaryschools, GP surgeries, libraries, sports facilities, open space, food shopping and local town centres.
Like the proposed CYTAL methodology we still need to agree a number of assumptions based on existing(TfL’s London Travel Demand Survey LTDS) or commissioned survey results. These include:

average walk speed – should we consider different walk speeds for different user classesmaximum walk distances to individual services



actual and perceived distance.

LTDS outputs show that the average “walk all the way” trip length is 0.65 km across London and the
majority (82%) of such trips are less than a kilometre in length.

Potential usage
We have already produced exploratory analysis calculating walk distance to a number of key locations:town centres and stations. An example output is given in Figure 25 showing walking distance to town
centres. The shaded areas show high, medium and low access based on distance. The 960 m interval
relates to the maximum walk distance used in the PTAL methodology. The 1 500 m interval was
considered an acceptable maximum distance for this analysis but this will need to be supported byfurther field work to establish appropriate intervals.
Similar analysis has also been used to investigate how the ATOS datasets (based on walking only) could
be combined with PTAL data. Figure 26 divides walk times to food shopping locations (defined as thenearest supermarket or town centre) and PTAL scores into two categories each. PTAL levels 4, to 6 andPTAL levels 0 to 3. Good access to food shopping is defined as within 15-minute walk time from any
location in London, whilst poor access is more than a 15 minute walk.
Combining and plotting these results identifies four distinct areasArea A: PTAL 4 to 6 with good access to local food shopping – dark red
Area B: PTAL 4 to 6 with poor access to local food shopping – pink
Area C: PTAL 0 to 3 with good access to local food shopping – light blue
Area D: PTAL 0 to 3 with poor access to local food shopping – dark blue

Of particular interest is Area C. In these locations there could be the potential for prioritising policy
interventions such as those related to TfL’s “Healthy Streets” objectives by encouraging more walking
trips to local services.

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Figure 25. Walking distance to town centres in London

Figure 26. Combining PTAL and walking access to food shopping

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Conclusions: Challenges and opportunities
By means of a conclusion, this section presents some of the challenges and opportunities that havearisen whilst developing TfL’s suite of connectivity tools and WebCAT. It also includes some of the topics
raised at the ITC workshop on “Improving planning and investment through the use of accessibility
indices”. We do not necessarily provide the answers but instead provide topics for further discussion.

Communicating technical concepts to a non-technical audience
WebCAT presents a good example of how TfL’s travel time outputs, derived from complex strategic
assignment models, have been made accessible and relevant to both a technical and non-technicalaudience. Time Out magazine recognised that “it’s used for planners to look at how well areas areconnected when planning new buildings. But it’s actually a surprisingly useable tool.”4
However, the website does include measures that require more complex interpretation such as ATOS
and catchment analysis. Furthermore, we have found it difficult to develop simple measures that define,for example, how cycling connectivity changes with infrastructure investment. Traditional measures rely
on travel time improvement whereas cycling investment typically leads to better quality infrastructure
rather than reduced journey times directly. Without using complex measures that trade off various
ambience characteristics associated with cycling, it will be difficult to develop meaningful measures.
Discussion question: What is the right balance between providing technical detail and complex measures
vs ease of interpretation? Which are the key audiences for these types of measures?

Updating our datasets
The transport network datasets used in our connectivity work are derived from our strategictransportation models. The advantage of this is that networks describing future conditions can beproduced with limited effort and on a consistent basis to the current year networks. However, thelimitation of this approach is that models are updated on a two-three year cycle and so can become outof date quickly, especially where the network is changing significantly.
Discussion question: Is there a benefit to using more real time data to base our connectivity analysis on?
What could these datasets be and do the benefits outweigh the cost?

Private modes/highway times
Currently WebCAT does not include private modes of transport. Although it is relatively easy to generatehighway and public transport travel time matrices, it is more difficult to provide an acceptablecomparison as both operate under different assumptions. For example parking search time andavailability is not included within private mode travel time but could be a significant factor in somelocations. Furthermore the public transport travel times we use for our connectivity analysis and in
WebCAT are based on real/non-generalised time (which does not take crowding and associated factors
into account), whilst the value of our highway data is that they are based on congested times.

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Discussion question: How important is inclusion of highway measures? How can we mitigate concerns
around comparability of public and private modes? How useful are highway measures in promoting theuse of sustainable transport?

Connectivity is not just about journey time
TfL’s travel time connectivity measures are in most cases based on fastest journey time. However, formany users this is not always the key issue. Journey complexity for example, reducing the need tointerchange is often a more important factor. A longer journey that does not require interchange toanother service is often preferable to one that is faster. Our step-free analysis has considered some ofthese issues but TfL will need to expand this analysis further and produce agreed outputs.
Another significant constraint to connectivity experienced by some users is the cost of travel especiallywhen getting to work in central London in the morning peak. WebCAT includes bus as a separate mode
which goes someway to accommodate for this issue but TfL will need to investigate further how fares
could be incorporated into an indicator that is both transparent and intuitive.
Discussion question: How useful are measures that build in non-journey time attributes? Which use cases
would this be required for? What are the associated challenges?

Formalised connectivity indicators vs ad hoc analysis
With WebCAT, TfL has endeavoured to offer relatively simple functionality that is of relevance to a wide
range of users: professional planners as well as the general public. The comprehensive list of WebCAT
datasets demonstrates the power and flexibility of the application in terms of the user being able totailor analysis to fit their individual requirements. Except for the PTAL measure, TfL has not provided any
formal indicators that quantify good or bad connectivity, leaving this interpretation to the individual user
based on their knowledge of the location chosen and the purpose of the analysis. For example aresidential property developer could generate a travel time map (based on all modes from the location)and employment catchment statistics to demonstrate how many jobs could be reached from a new
housing development. Alternatively a commercial developer may be more interested in how manypotential customers could reach a new retail site within the same area. Thus both users require different
analysis for potentially the same location.
Discussion questions: Are formalised connectivity indicators required for all circumstances? Should wedevelop more flexible tools that allow users to undertake their own analysis within agreed parameters?

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Notes

1 Mayors Transport Strategy 2017 (https://www.london.gov.uk/what-we-do/transport/our-vision-transport/draft-mayors-transport-strategy2017)
2 https://www.london.gov.uk/what-we-do/transport/our-vision-transport/draft-mayors-transport-strategy-2017
3 www.tfl.gov.uk/WebCAT
4 https://www.timeout.com/london/blog/tfl-has-a-tool-that-works-outhow-long-it-takes-to-travel-to-anywhere-in-london© OECD/ITF 201837

London’s Accessibility Indicators:
Strengths, Weaknesses, Challenges

International Transport Forum
2 rue André Pascal
F-75775 Paris Cedex 16
T +33 (0)1 45 24 97 10
F +33 (0)1 45 24 13 22
contact@itf-oecd.org
www.itf-oecd.org

2018-09/Photo credit: Von Kanuman/|Shutterstock

The paper provides an introduction to London’s context and the needfor accessibility indicators. It overviews existing indicators developed
and used by Transport for London, including: PTAL, ATOS (Accessto Services) and catchment- based measures, as well as giving an
introduction to TfL’s online web portal for connectivity assessment:
WebCAT. There is also a review of calculation principles, a summaryof user cases and an analysis of the strengths and weaknesses ofeach method. A second section describes indicators (e.g. PTAL
incorporating cycling, and walking catchment analysis) that are
currently under development as part of TfL’s commitment towards
prioritising healthy streets and sustainable modes of transport.
The final section focuses on challenges and opportunities identified
during the development of these indicators.