Updated Guide to Traffic Impact Assessment

Ilona Blackburn - Principal Consultant—Traffic | RPEQ

In July of this year TMR released The Guide to Traffic Impact Assessment (GTIA) which replaces the former Department of Main Roads’ Guidelines for Assessment of Road Impacts of Development (GARID), 2006.

The new guide places clear emphasis on safety and efficiency of the road network and intends to make the assessment process more relevant to each individual development, in regards to responsibilities for contributions and implementation of mitigation strategies.

The format of the GTIA is based on the following six guiding principles.

1 – Development must not compromise safety on the SCR (State Controlled Road) network.

Safety is paramount in the road environment and TMR seeks to ensure adequate levels of safety for all users on the SCR network. Accordingly, TMR would not permit any development outcome that would adversely impact road safety (for example a development that increases the likelihood or severity of crashes with the potential to result in a fatality or serious injury) without commensurate mitigation works or road-use management strategies.

2 – Development should seek to achieve no worsening to safety or infrastructure conditions and no net worsening to efficiency across the network.

The principles of ‘no worsening’ and ‘no net worsening’ aim to ensure that the current and forecast characteristics of the transport network are not significantly worse than the current and forecast characteristics existing without the development.

3 – Development should seek to adopt a mitigation hierarchy of avoiding impacts, managing impacts and mitigating impacts.

Development should always seek to avoid or reduce impacts on the network where possible. Where adoption of avoidance strategies leaves residual impacts, these impacts should be managed. Where this is insufficient, impacts should be mitigated through infrastructure works, or where there is no alternative, a monetary contribution equal to the value of the mitigation works.

4 – Development access locations and permitted turning movements are consistent with the function and access limitation requirements of the road being accessed.

The primary function of most SCRs is to carry through traffic including freight vehicles. Additional property access points onto SCRs can compromise this function, particularly where right turn movements are allowed. Development should minimise traffic access and/or allowable turning movements at SCRs where alternative, safe, legal points of access are able to be provided (i.e. if a site fronts both an SCR and Local Government road – access should be provided from the Local Government road where available).

5 – Development proponents are responsible for all access and frontage works.

Access and frontage works includes all roadworks required for a direct connection between the development site and the SCR network, and may also include works required as result of impacts on pedestrian and cycling infrastructure, public transport infrastructure, kerb and channeling and storm water infrastructure.

6 – Development proponents are responsible for all works required to connect out of sequence developments to the current and future SCR network, whilst not compromising the implementation of planned transport infrastructure.

TMR has little control over the type, location and timing of development and given budgetary restraints, focusses its investment program on regional priorities rather than supporting individual projects. Proponents of development that is out of sequence or unanticipated are responsible for all infrastructure required to connect to the SCR network. Out of sequence development must also plan for and take into account planned transport infrastructure identified in planning studies that may not yet be committed for construction.

There are positive changes to the process adopted to assessing traffic impact of developments. A very noticeable change is the impact assessment years. Under the GARID we were required to assess the opening year and the design horizon of opening year plus 10 years for all intersections/links within the assessment area. This leads to a number of issues, particularly when it is the background growth, not the development, which apparently “breaks” the analysis model. An impact to which the development ultimately may be required to contribute for as the intersection essentially cannot accommodate the impact – even though it is not directly attributable to the development.

The new guide moves away from this – in that only the access (or accesses) will need to be assessed in this manner – which is a much fairer process as the development that wholly relies on this and therefore should be responsible for its implementation. On the wider network we are required to assess opening year (by stage) only – i.e. no 10 year design horizon.

Whilst this may reduce the amount of SIDRA analyses we undertake, it may increase the overall data analysis as we take into more information about the wider environment as we consider the type, number, frequency and severity of accidents, address link capacity both upstream and downstream of the development and provide qualitative reasoning of developments impacts.

This additional data analysis encourages us to demonstrate the steps we have taken to get to the solution we provide – to satisfy the guiding principles. This is a welcome and common-sense approach allowing us as traffic engineers to inform our clients in the early stages of a project – rather than being tasked to squeeze in a solution without impacting on the built form. It will allow us to provide the assessing authority with all the facts and background as to why the development form is the way it is as we are providing an informed solution that has taken into account the wider local environment.

In the past a common approach was to assess the impact and mitigate it – with little or no regard as to the suitability of the mitigation design. TTM welcome the requirement to provide valid reasoning for why the development cannot avoid the impact or manage the impact on the public road network, particularly if we are proposing to mitigate the impact through significant intersection upgrades or road widening. The intent being to reduce the quantity of ad hoc upgrades and disruption on the network through smart and informed design solutions.

For further information or advice on how the new guide informs your project, please get in touch with the TTM team.