Scottish Government

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.