Introduction
A contractor responsible for upgrading a state highway intersection faced a complex challenge: a long-term diversion through narrow geometry, a hazardous fence post near the carriageway edge, and chicane-like entry points doubling as site access for heavy vehicles. Instead of defaulting to a two-lane approach, the contractor commissioned a more rigorous risk-based design to see whether a more straightforward single-lane diversion, managed by temporary traffic signals, could keep workers and road users safest. This case study demonstrates how an independent, evidence-driven TTM design—aligned with New Zealand’s shift toward risk-based TTM—brought tangible improvements in safety, scheduling, and overall project efficiency.
Why This Matters
New Zealand’s TTM sector is under growing scrutiny for the perceived overuse of cones, spiralling TTM costs, and longstanding reliance on prescriptive guidance. System reforms, public frustration, and urgent calls for better value have accelerated a nationwide pivot toward risk-based traffic management. By decoupling Efficient TTM Plan from TTM delivery and investing in thorough hazard analysis, this contractor not only cut down on unwarranted equipment and labour but also validated a design that better fit the actual conditions of the site.
The Project Context
Over several months, the temporary alignment around a new intersection would form the main traffic path on a state highway. Conventional wisdom said two narrow lanes, delineated by cones down the centre, could accommodate the traffic flow. However, this approach left minimal shoulders, forced labour-intensive maintenance of centerline cones, and provided little room to shift the running lanes for excavation or servicing utility trenches nearby. A wooden fence post along the carriageway edge threatened any vehicle that strayed too close. At each end, chicane-like merges and site access points had to be managed so road users did not accidentally follow construction vehicles into the work zone.
Parallaxx was engaged to evaluate options. Was it truly safer—or even necessary—to maintain two lanes through this zone? Should a road safety barrier system be installed to protect against the fence post? Or was there a leaner, risk-based alternative that would result in lower risk?
Comparative Risk Assessment
A bottom-up qualitative risk assessment examined two main approaches:
1. Two-Lane Diversion
- Two lanes of traffic squeezed into the same corridor.
- Cones or delineators along the centerline.
- Minimal shoulders, leaving little safe space for on-foot TTM staff or variable working widths.
2. Single-Lane Altering Flow
- One open lane, centrally located to create wider shoulders on either side.
- Temporary traffic signals at both ends, releasing northbound and southbound traffic in alternating phases.
- Flexibility to slightly “slide” the lane position if the ground surface or adjacent work required it.
The assessment also evaluated whether a road safety barrier system offered significant risk reduction, given the moderate speeds (generally <50 km/h) and ability to halt traffic when workers were close to the live lane.
Single-Lane Flow Advantages
- Controllable Access
Traffic signals offered a reliable way to force an all-red phase while heavy trucks or equipment moved in or out. That blocked the public from inadvertently tailing work vehicles into hazardous zones.
- Speed Reset
Requiring drivers to stop at each end kept speeds down on the rough pavement. At 50 km/h or less, occupant injury risks decrease markedly.
- Wider Shoulders
Placing the single lane in the centre freed up 1–1.5 m on each side, reducing side-swipe chances, simplifying TTM maintenance, and letting crews make small lateral shifts as needed.
- Less On-Foot Exposure
Absent a continuous centerline of cones or barriers, TTM staff had fewer occasions to step into live traffic for maintenance.
Potential Compromises
- Queueing & Drive-Offs
Single-lane flow could generate multi-minute waits at red signals, tempting impatient drivers to run them. Rear-end collisions were also a risk if queues grew too long.
- Added Controls Mean New Hazards
Solutions—like a variable message sign (VMS) to show wait times—could themselves become roadside hazards unless placed safely and included in the risk analysis.
Road Safety Barriers: Necessary or Not?
The next step was to examine whether a barrier system around the fence post or the entire diversion length would appreciably drop residual risk. The findings were instructive:
- Operating Speed & Space
Speed would hover around 50 km/h, and the single-lane design already created wide shoulders. Any collision with the fence post at such speeds was less severe than at open-highway velocity.
- Workability & Schedule
Installing and shifting road safety barriers through multiple construction stages would consume time, block easy site access, and reduce the ability to reposition the lane. A preliminary estimate suggested that an extra month of work would be lost due to barrier installation, removal, relocation, and constraints on the working cadence.
- Minimal Net Benefit
With signals to halt traffic whenever crews worked near the live lane, the barrier delivered negligible extra protection for workers, and occupant risk from the post was not materially lessened at this moderate speed.
It was determined that a full-length barrier system would hinder daily operations while offering a scant additional reduction in the risk profile.
Outcome: Saving an Entire Month
One entire stage was eliminated in the traffic management program by opting for the single-lane approach. An overall one-month reduction in site occupation equated to fewer daily hazards, direct savings in TTM gear and labour, and accelerated project completion. Weekly TTM overhead dropped roughly 20%. The initial expense of more detailed planning (including traffic modelling to confirm queue lengths and signal phases) proved minor against the aggregate savings in time and equipment.
Lessons for an efficient TTM plan
- Risk-Based Planning Can Shrink On-Site Footprint – Aligning Efficient TTM plan controls more closely to the risks avoids spill-over that can inflate cost and complexity. More stuff is not always better.
- Time is Part of the Risk Equation – Shortening the overall program has a direct correlation to overall risk magnitude.
- Barriers Are Not Always a Silver Bullet – In moderately low-speed settings with controllable traffic, a barrier’s net effect may be outweighed by lost flexibility (impacting time) and added installation and removal risks.
- Worker Safety Improves with Targeted Controls – Fewer cones in the live lane and the ability to stop traffic on demand protect TTM crews better than continuous manual setups.
- Decoupling Design from Delivery – When TTM planning is done independently, each measure must prove its worth. The incentive to oversupply is gone.
Where Th is Fits in New Zealand’s Broader Reforms
Amid shifting expectations about TTM spending and public impatience at “cone overload,” this case provides an example of an evidence-driven design that simultaneously lowers cost, time on site, and safety risks. It supports the national transformation from prescriptive frameworks to a deeper level of risk-based thinking. This project underscores the growing momentum behind independent, risk-based TTM design by showing how extra consideration and risk assessment regarding TTM methods can have multi-faceted value.
In short, leaning on thorough risk management processes for TTM allowed the contractor to finish the job an entire month faster.
The broader implication is that an efficient TTM plan, decoupled from delivery incentives and validated by data, aligns strongly with safe, streamlined, and cost-conscious practices that genuinely reflect the context of each situation.
