New changes for temporary traffic management – lowest total risk

October 29 2024

The NZ Transport Agency will no longer be supporting or maintaining the Code of Practice for Temporary Traffic Management (CoPTTM). Temporary traffic management (TTM) for state highways is now required to be in accordance with the New Zealand Guide to Temporary Traffic Management (NZGTTM).

An ACE New Zealand TTM reference group has been formed to support the professional services community with the changes from a compliance-based to a risk-based approach. This thought piece written by the group summarises the changes to TTM.

CoPTTM refers to managing risk and highlights that the TTM solutions described in the code are the minimum criteria.  However, risk assessment and management have not typically been an integral part of the delivery of TTM on our road networks. Instead, the prescriptive approach presented in CoPTTM has been followed.

One of the fundamental principles of the NZGTTM is that TTM solutions should result in the lowest total risk. The guide states: “Sometimes a control for one risk may increase or transfer risk to another group of people or it may introduce another risk. This is where the concept of lowest total risk applies.” 

This thought piece does not provide advice on methods to be used for analysing risks and identifying the lowest total risk. However, it encourages readers to recognise that risk comes in a variety of forms and, while a solution may minimise risk for one group, it may result in significantly increased risks for another group.  Hence the importance of TTM solutions resulting in the lowest total risk.

The NZGTTM library provides this guide to assist practitioners with considering the lowest total risk. However, it is important to consider the various aspects of risk and to recognise that road users have access to a wide variety of information sources, not just those that we provide. There is a wide range of risk types, including:

  • Safety
  • Financial
  • Logistics
  • Health
  • Environmental
  • Operational
  • Reputational

Road closure case study – lowest total risk

Closing a section of road to remove normal traffic from that location should improve the safety of a construction or maintenance operation within the closed section, and may also result in lower financial, health, environmental, and operational risks for the activity itself.  However, closing the section of road means the normal traffic using the road needs to be detoured to other routes.  While a TTM solution may propose specific detour routes and we can analyse the risk associated with those routes, we must also remember that road users will identify other predictable detour routes. Therefore, assessment of the total risk must consider the reasonably predictable risks associated with the TTM solution.

The first image below provides an example of a road closure and the associated intended detour routes. For perspective, the section of road that has been closed has an average annual daily traffic (AADT) ranging from 7,600 to 10,200 vehicles per day. The light vehicle detour route currently carries an AADT of 550 vehicles per day on SH30, and 1830 to 2250 vehicles per day on SH32. Therefore, the detour solution proposes to very significantly increase traffic volumes on SH30 and SH32. There are risks associated with this.

Light and heavy vehicle detour routes associated with closure of State Highway 1 (image source: NZ Herald: 4 October 2024)

However, it is probable that the journeys taken by road users are not limited to an origin (or destination) immediately adjacent to one end of the road closure with the associated destination (or origin) being immediately adjacent to the other end of the road closure.  Therefore, it is predictable that road users will obtain information from sources such as Google Maps, which may put them on unintended (but predictable) detour routes that are less suitable for the detoured traffic than those routes onto which the TTM solution proposes for the detoured traffic. 

The image below is a Google Maps solution for a journey from Taupo to Hamilton. While it shows the road closure location on SH1, the route along which road users are guided by Google Maps is significantly different to (and much less suitable than) the route road users are more likely to follow when the road closure is not in place.

Detour route as identified by Google Maps (11 October 2024)

According to the NZ Transport Agency MegaMaps, the SH32 portion of the light vehicle detour route from Whakamaru to Tokoroa has a medium (personal) risk.  However, sections of the Google Maps route through to Te Awamutu are riskier on a personal journey basis being medium-high and high risk. This information should be a contributing factor to the assessment of the lowest total risk. Consideration should be given to what may need to be done to manage risks on the alternative routes.

The total risk analysis for the TTM should include consideration of predictable scenarios such as a crash occurring on the detour route.  For example, firstly, what are the increased risks that result from detoured traffic being involved in a crash? And, secondly, if the intended detour route is obstructed due to an incident, what are the secondary risks?

Key takeaway

The key message from this thought piece is that the NZGTTM approach for identifying the most appropriate TTM solution to facilitate works being carried out needs to identify the lowest total risk.  The analysis of that risk needs to consider all types of risk and all reasonably predictable consequences associated with the TTM solution identified.  As we move into the summer construction season, it is important that ACE members involved with developing and assessing TTM solutions consider the lowest total risk, not just the risks directly associated with the work and an assumption that everything else will be fine.

Want to find out more?

The ACE NZ TTM reference group, that has been formed to support the professional services community with the changes from a compliance-based approach to a risk-based approach, meets monthly to stay up to date with the developments across the sector and explore matters relevant to ACE members. If you would like to chat with one of the reference group members, please feel free to contact them below: