Planning Portal

Transport Assessment
Stage 3: Appraising and Mitigating Impacts: Highway and traffic impacts

The Transport Assessment must cover traffic and highway issues and parking.
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.
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.
When increases in road capacity are considered necessary and acceptable the design should give priority to walking, cycling and public transport.

Traffic impact analysis

Transport Assessments must identify the volume, distribution and assignment of vehicle trips related to the development and set this within the context of existing traffic movements in the locality. The guidance on traffic impact analysis referred to sets out best 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 road will be.
  • Principles for establishing the acceptability of mitigation measures for development impacts will be advised by Roads Service during scoping discussions. These principles, which will relate to the importance of the road junctions and/or links within the road network and to the current level of congestion, include:
    • Maintaining 10% reserve capacity at the design year; or
    • Providing for 5 years growth; or
    • Restoring the flow/capacity ratio to the level prior to the development traffic.
Normally, it would be expected that the road network would not experience any net detriment at the design year following the implementation of the development proposal.
  • 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; and
  • 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.
Whilst 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.
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. Micro-simulation software is developing and may provide useful tools.
More traditional methods may be pertinent. They:
  • 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.
Supplementary guidance relating to various aspects of traffic impact analysis is included in Appendix D to assist developers and consultants in understanding Roads Service’s technical requirements. The Roads Service document, Transport Assessment/Traffic Impact Assessment Guidelines, published in 2001, has been superseded by these guidelines.
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