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APPENDIX A: TRANSPORT ASSESSMENT: HOW TO ASSESS
THE SITE AND ITS IMPACTS
A. The transport assessment in detail
The Site Visit
A.1 The role of the Transport Assessment is to provide
decision makers with a good understanding of how the
transport aspects of the development will function. The
first stage to be undertaken, either in association with or
before the scoping discussions, is a site visit.
A.2 One or more site visits may be needed to examine
aspects such as:
- accessibility within the site boundaries. For
larger developments it will be necessary to consider
the travel time across the
functional area of the development: from the
boundary of the site with public access routes to the
'entrance' of the building(s). The site visit may
suggest other layouts for the building/s and locations
of doors and entrances to the site which could save
time and provide better quality access by non-car
modes;
- accessibility on foot to the site, including for
those with mobility impairments, from the surrounding
locality, bus stops and railway stations;
- pedestrian crossings and safety, including whether
alternative layouts on the surrounding roads might
reduce conflicts between pedestrians and motorised
traffic or cycles;
- access by bicycle, identifying opportunities for
improving cycle access including cycle lanes, junction
and crossing improvements;
- access from bus stops (railway stations),
calculating walking times from bus stops (or stations)
and considering whether new bus stops are required or
existing ones need to be relocated; and
- access for vehicular traffic (buses, if
appropriate, cars and lorries), identifying potential
conflicts with non-vehicular modes, as well as possible
local congestion problems.
Measurement of Site Accessibility
A.3 There are various measures of accessibility and
methods of calculation. Determining the accessibility of a
site will require calculating the travel time by different
modes of access: walking, cycling, public transport and
car. Travel time assessments determine the catchment area
of a development by different modes: areas within which one
can reach a development within set times or time-bands
(e.g. 30 mins). Catchment areas for a location can be shown
in isochrones on maps. The choice of time-band may vary in
response to the use and scale of the development. People
may be prepared to travel further for some activities, for
example, to a sports stadium than to a shop.
A.4 Journey times of 20-30 mins are appropriate for
walking and 30-40 mins for cycling. A two-stage process is
recommended: estimating time to the development site by
analysis of maps; then checking the actual times of people
travelling these routes, which will help take account of
factors such as the time required to cross roads or
walk/cycle up hills.
A.5 Public transport journey times can be calculated by
a combination of analysis of timetables and maps. This
should be complemented by observation of walking times to
actual (or potential) bus stops. A 30 minute door to door
travel time (including the walk, wait, journey time, and
walk to the destination) is an appropriate choice of
time-band by public transport for most types of development
although it may also be helpful to consider a 45 minute
door to door travel time. For developments of national or
regional importance, 1 hour may be appropriate.
A.6 Car access can be calculated in various ways,
including analysis of maps and route planning software. It
should include estimates of the in-vehicle travel time
together with walking to the vehicle, searching for a
parking space, and walking from the vehicle to the site
entrance plus likelihood of known congestion on route. Time
estimates for other motorised vehicles - motorcycles,
mopeds, light vans,
HGVs - can be assumed to be the
same.
A.7 For housing developments a different approach is
needed since it is an origin rather than a destination for
journeys. The travel time assessment should measure the
time taken to reach services (e.g. shops, employment
centres) from the housing development, but the basic
methods used will be the same. Travel times across the
development site for larger housing developments may be
significant, and separate analyses may be needed for
different parts of a very large development.
Estimating the Travel Generated and Likely Mode
Share
A.8 The estimation of how many people will travel to the
site (or in the case of housing, from the site) and by what
mode requires consideration of the:
- location of the site;
- how many people are living within the travel time
isochrones by each mode;
- whether there are competing developments that will
significantly affect the catchment area;
- the likely propensity of people within each
catchment to use the proposed facility;
- larger catchment areas (often for larger
developments) imply a higher modal share for car-use
but also offer more potential for public transport
use;
- whether people need to carry bulky items to or from
it, such as for
DIY stores;
- whether people will be likely to visit the site as
part of a linked-trip to other locations, for example
for pass-by shopping;
- how design and layout within the site helps or
hinders access by different modes and adds to or
reduces travel times; and
- measures taken to influence modal split and how
they are likely to influence the choice of mode.
Available Databases
A.9 Estimating travel generation and mode share requires
appropriate data. However the suitability of data for
Transport Assessments is of variable quality, with much
depending on the location where the development is
proposed, and the resources available to the developer
undertaking the assessment. Since analysis can only be, at
best, as good as the data on which it is based, there is an
ongoing need to collect and use good quality data.
Appendix B describes the data
available in Scotland.
Integration
A.10 Transport policy emphasises the need to integrate
the different modes of transport, and development proposals
provide opportunities for achieving this objective.
Transport Assessment and Implementation documents should
clearly identify how the proposal will influence
interchange between modes in the area. Where large flows of
people are forecast there may well be impacts on the
efficiency of interchanges, and for developments such as
arenas and stadia these may affect interchanges at some
distance from the proposed site. Many of the improvements
to foot, cycle and public transport described above will
help address any identified problems.
A.11 The Transport Assessment should also identify how
the proposed development would affect activities within the
local area. For instance, mixed-use development may provide
for linked trips without the need for additional car
journeys. Some developments may mean that certain functions
are within walking distance where previously a journey out
of the area would have been needed. The opposite can also
apply, particularly in relation to single-use car
orientated development, such as business parks located on a
by-pass.
Safety
A.12 The two main areas of impacts that should be
assessed are:
- the risk of traffic related accidents for those
using and passing by the site; and
- feelings of insecurity for those using and passing
by the site.
A.13 Changes in the risk of accidents result from
changes to the volume and mix of traffic, the layout of
footways, cycle-ways and roadways, and accesses to
roadways. These can be appraised before the introduction of
the development by means of a safety audit. For
developments on trunk roads this is a legal
requirement.
A.14 The most direct indicator of safety is the number
of recorded accidents. For larger developments it may be
possible to predict likely impacts on the number of
accidents by considering data relating to accidents on
different types of road and junction types.
A.15 For smaller developments this is usually not
possible, nor does it cover all aspects of safety relating
to accidents. In these situations it is necessary to look
for design factors which are likely to lead to conflict
between different users.
A.16 A range of design and social factors determine
perceptions of risk and personal security depending on the
characteristics of areas that pedestrians use. Design
factors include:
- characteristics of site perimeters (such as whether
solid walls are used, and the design features of
entrances and exits);
- surveillance systems (such as
CCTV, and staff with the role of
surveillance);
- informal surveillance (relating to visibility lines
from busy areas);
- landscaping (relating to visibility, and 'cover'
for intruders);
- lighting and visibility (such as placement of
pillars, recesses, and quality of lighting);
- the provision of emergency call facilities;
- how busy the area feels: a greater presence of
people leads to greater feelings of security; and
- good sight lines and a lack of 'dead ends'.
Environmental Impacts
A.17 The environmental impacts of a development proposal
are generally outside the remit of the transport assessment
process, as they should be picked up through an
Environmental Impact Assessment (
EIA). For some types of development an
EIA is always required; for others it is
required if the planning authority considers that the
development is likely to have significant effects on the
environment. Where both an
EIA and a transport assessment and
implementation report are required, usually for a very
large development, the Transport Assessment stage of the
report would provide much of the transport-related
information needed for the
EIA.
A.18 In some cases, the local authority may think it is
appropriate that the report covers one or more specific
environmental issues:
- Noise levels: if the development is
likely to generate significant levels of additional
traffic, an estimation of the impact upon local noise
levels may be necessary.
- Local air quality: for developments
generating significant levels of additional transport,
the local authority may consider that an estimation of
the impact upon local air quality should be
incorporated within the Transport Assessment.
- Landscape, townscape and heritage
impacts caused by transport would normally be
part of the planning application as a whole. However,
the local authority might consider it necessary for the
Transport Assessment to provide a particular focus on
certain of these issues, to avoid the need for separate
studies.
Road and Traffic Impacts
A.19 Transport Assessment must cover traffic and road
issues, parking and any particular impacts caused by
abnormal loads.
A.20 While a key aim is to promote access by sustainable
modes and to reduce car dependency, there will in many
cases still be road and traffic impacts to address and deal
with as part of the planning application. However, it is
particularly important to ensure that all the ways to
promote sustainable modes and reduce car-use have been
fully explored and utilised. Other traffic management
measures should also be considered before looking to
increase road capacity.
A.21 If an initial assessment of the proposal indicates
that the predicted traffic levels are still unacceptably
high, it should indicate a need to re-consider whether
further measures to reduce the level of traffic generation
are necessary. If after further consideration, the proposal
illustrates that considerable extra road capacity will
still be required to accommodate predicted traffic
increases, the local authority may need to consider
reducing the scale of the development or refusing planning
permission.
A.22 When increases in road capacity are considered
necessary and acceptable, the design should give adequate
priority to walking, cycling and public transport. Such
measures should also be consistent with the Local Transport
Strategy.
Traffic Impact Analysis (
TIA)
A.23 Transport Assessments must identify both the volume
and distribution of vehicle trips related to the
development and set this within the context of existing
traffic movements in the locality. This is the element of
the process most closely related to
TIAs. The guidance on
TIAs (
IHT, 1994) sets out the practice in this
area, but the following should be noted:
- Extent of the Transport Assessment
should be sufficient to identify significant traffic
effects. These impacts may be some distance from the
development.
- The significance of a traffic impact
depends not only on the percentage increase of traffic
but the available capacity. A 10% increase on a lightly
trafficked road may not be significant, whereas a 1%
increase on a congested motorway will be.
- Design dates for appraisal should
generally be for shortly after opening, within a year,
especially for retail and employment uses, or on
completion of the development in the cases where the
development is large and phased over a long period of
time (e.g. large residential developments). The
susceptibility of infrastructure and services to growth
should be clear from examination of the proximity to
design thresholds. Some developments and their
infrastructure requirements will be of such
significance that a longer term design date may be
demanded. Developers should seek clarification on this
issue from the roads authority at an early stage in the
process.
- Phasing of development should also be
taken into account. In the case of housing, this may
require testing at a number of future dates to align
transport provision with increasing demands. This could
also link with a timetable for developer
contributions.
- Future effects of other measures to
increase travel by non-car modes should be taken into
account. These may form part of a planning agreement or
Travel Plan due to be implemented over time.
- Catchment and locational features
should be clearly related to trip generation
assumptions. Whereas the size of the catchment area
will determine potential traffic generation, the
location will determine the level of diverted and
pass-by traffic.
- Retail impacts can be complex. Account
should be taken of the potential for growth in some
retail markets (e.g. non-food) but not in others (e.g.
food). Retail developments can influence trip-making as
markets mature, but this depends on the scale and
catchment of the store.
A.24 In most cases, complex calculations as above will
not be required since the impacts of most new developments
are usually very localised. This will not necessarily be
the case for residential and the larger commercial and
mixed-use developments. Their traffic impacts must
therefore be assessed over a larger area.
A.25 Whilst road traffic impact analysis should focus on
peak periods, in line with current junction-testing
techniques, the effects of peak spreading and the impact
during inter-peak periods should not be ignored. The
Transport Assessment should indicate days and times when
the combination of development and non-development traffic
will peak. Daily travel information and traffic time
profiles are useful in the following areas:
- identifying busy hours for testing;
- assessing bus and rail service viability; and
- assessing car parking accumulations over time.
A.26 The models and procedures for testing the effect of
traffic levels are not expected to change significantly,
since they are based on the way traffic is observed to
flow. However, an exception is the use of micro-simulation
software, which is still developing and involves
incorporating junction assessment techniques into a wider
representation of network operations. Microsimulation
models are often used for the analysis of the roads impacts
of development. Some have the ability to model bus priority
and even air quality and can have a useful role in public
consultation.
A.27 More traditional models:
- focus on road traffic impacts, so it is important
not to let them deflect attention from provision for
other modes;
- need to be validated against current traffic
behaviour (such as link and turning flows, queues and
delays, etc.) before they are used to predict trends;
but
- are particularly useful where a major change to
traffic movements is contemplated (which is fairly
rare).
A.28 It is important to recognise that where models are
to be applied to detailed development-related traffic
issues the models must be "fit for purpose". An area-wide
model validated across that area to
DMRB standards may not be appropriate
for specific corridor testing unless that corridor
validates to the required standard.
Parking Impact Analysis
A.29 On-site parking provision should conform to demand
management principles and be compatible with the policy
guidance on parking set out in
SPP 17 and levels stated by the local
authority, particularly in the Local Transport Strategy and
local and town centre parking strategies.
A.30 Transport Assessment should demonstrate how the
need for parking has been minimised in new development and
redevelopment. It is no longer appropriate to focus on
providing sufficient parking to satisfy all demand.
Over-provision of parking is still common in development
proposals, largely based around the routine use of the 85th
percentile in car trip rate assumptions. A more appropriate
trip rate choice will therefore lead to more realistic
parking provision. Such an approach will often be of
benefit to developers, who may then be able to increase the
density of the development.
A.31 The analysis of parking should focus on the
requirement for parking as an output of the design of the
development, once other measures have been fully taken into
account. It should highlight whether there is potential to
level the peaks of parking demand, for instance through
shared use of spaces between parts of the development which
have different peaks in demand. On-site parking controls
and charges may also need to be introduced.
A.32 Off-site parking provision and controls need to be
included in the Transport Assessment stage and reflected in
the other areas of the report. Restrictions on on-site
parking may lead to overflow parking in the surrounding
area. Development proposals may need to contribute towards
the introduction of on-street parking controls, for
instance for a residents' parking scheme, as part of the
overall package of measures associated with an
application.
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