Scottish Government

Guide to Transport Assessment in Scotland

PART 2 - TRANSPORT ASSESSMENT: HOW TO ASSESS THE SITE

9. THE DEVELOPMENT SITE

Site Visit

9.1 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, possibly before the scoping discussions, is a site visit.

9.2 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 highways 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 Accessibility to and from the Site

9.3 There are various measures of accessibility and methods of calculating it ². 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 line with 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.

9.4 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.

9.5 Public transport can be calculated by a combination of analysis of timetables and maps. However this should be complemented by observation of walking times to actual (or potential) bus stops. A 30 minutes 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.

9.6 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.

9.7 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

9.8 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 including:
• 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; and
• 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.
• use of the development, including:
• whether people need to carry bulky items to or from it, such as for DIY stores; and
• 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.
• measures taken to influence modal split and how they are likely to influence the choice of mode.

Available Databases

9.9 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 type of area that a development is proposed in, and the resources available to the developer undertaking the assessment. Data exists to analyse and predict modal split, but in many instances the resources and expertise required would be significant, and the accuracy of forecasts could be unreliable. ( Appendix B describes the data available in Scotland).

10. APPRAISING AND MITIGATING IMPACTS

10.1 Having assessed the travel characteristics of the development and the potential of the site, the Transport Assessment has to assess the impacts and if necessary devise approaches to mitigate these.

10.2 The Transport Assessment should consider impacts similar to those in the Scottish Transport Appraisal Guidance (STAG) (Scottish Executive, 2001) which has been developed to appraise major transport related schemes. While the objective of a Transport Assessment is to help promote more sustainable developments, the impacts considered need only focus on the transport-related impacts of a proposal: accessibility, integration, and safety. There may also be some cases where some specific environmental issues should also be covered (although these may be the subject of an EIA).

10.3 The summary of transport impacts and what measures are being taken to deal with them should be set out in the Transport Assessment Form ( Appendix A). It is important to make at least an initial assessment of transport impacts as early in the planning process as is possible so that any necessary changes to the proposal can be incorporated into the design of the scheme.

Accessibility

10.4 Impacts under accessibility and integration include access to transport systems, the local area and community severance; and integration between transport systems and with other land uses.

10.5 New developments can have both positive and negative impacts on accessibility (which may be intended or unintended) including access to transport systems, the local area and community focus and cohesion:

• access to transport systems: changes which should be identified and appraised as part of the Transport Assessment include: the location of access points and links by foot and cycle to the wider public transport and road networks; the creation of new public transport nodes or links provided to serve the development that will benefit others; access for freight to the road and rail networks;
• access to the local area: such changes should have been identified in the measurement of accessibility in the first part of the Transport Assessment. In most cases this should support the development (providing accessibility and social inclusion benefits to local people), but such analysis may highlight measures which can further enhance access to the local area; and
• community severance: it is possible that a development might simultaneously increase access to the site, whilst creating a barrier to access within the local community. The assessment should identify any effects which the proposed development could have on the cohesiveness of communities. These impacts include:
• whether the development itself or changes to transport infrastructure act as a barrier to movement and previously used routes;
• whether the level of traffic accessing the site or using nearby roads make links between parts of the community more difficult.

Integration

10.6 Transport policy emphasises the need to integrate the different modes of transport, and development proposals provide one opportunity for achieving this objective. Transport Assessments 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.

10.7 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

10.8 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.

10.9 Changes in the risk of accidents result from changes to the level 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 (see IHT, 1996). For developments on trunk roads this is a legal requirement.

10.10 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. It is based on data relating to accidents on different types of road, and junction designs.

10.11 For most smaller developments this is not suitable, and 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.

10.12 A range of design and social factors determines 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

10.13 The environmental impacts of a development proposal are generally outside the remit of a Transport Assessment, as they should be picked up through an 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 are required, usually for a very large development, the Transport Assessment would provide much of the transport-related information needed for the EIA.

10.14 In some cases, the local authority may think it is appropriate that the Transport Assessment 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.

Highway and Traffic Impacts

10.15 The Transport Assessment must cover traffic and highway issues, parking and any particular impacts caused by abnormal loads.

10.16 While a key aim is to promote access by sustainable modes and to reduce car dependency, there will in many cases still be highway 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 highway capacity.

10.17 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 of refusing planning permission. 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

10.18 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 old guidance on TIAs (IHT, 1994) sets out much 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. An obvious exception is major housing developments, because schemes may take some time before being fully occupied. For schemes affecting trunk roads and motorways, however, it will need to use a date 15 years after opening. The susceptibility of infrastructure and services to growth should be clear from examination of the proximity to design thresholds.
• 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 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 exert downward pressure on trip-making as markets mature, but this depends on the scale and catchment of the store.

10.19 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 larger developments and their traffic impacts must therefore be assessed over a larger area.

Highway Impact Analysis

10.20 Whilst highway 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.

10.21 The models and procedures for testing the effect of traffic levels are not expected to change significantly, since they are based on the ways 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. These techniques can require significant data inputs.

10.22 One such model, PARAMICS, is often used for the analysis of the highway impacts of development in Scotland. It also has the ability to model bus priority and even air quality and can have a useful role in public consultation. More traditional models also:

• 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).

Parking Impact Analysis

10.23 On-site parking provision should conform to demand management principles and be compatible with the policy guidance on parking set out in NPPG 17 particularly the Addendum; and levels stated by the local authority, potentially in the Transport Strategy and local and town centre parking strategies.

10.24 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 intensity of the development.

10.25 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 uses which have different peaks of demand. On-site parking controls and charges may also need to be introduced.

10.26 Off-site parking provision and controls need to be included in the Transport Assessment. 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. Such measures need to be fully included in the Transport Assessment

2. Scottish Executive research, a Review of Accessibility Analysis Techniques and their Application (contains practical guidance and a CD-ROM containing application software on the application of accessibility analysis to development planning and development control).