The Return on Risk
Growing evidence from controlled trials and real-world projects is rewriting what New Zealand’s traffic management industry knows about cost, safety, and the relationship between the two.
The assumption
For decades, the default approach to temporary traffic management in New Zealand has followed a simple principle: more control equals more safety.
More cones. More signs. More barriers. More prescriptive requirements. This thinking comes from a genuine concern for worker and public safety. And for a long time, nobody measured it.
New Zealand has been transitioning from CoPTTM — a highly prescriptive traffic management framework — to the NZGTTM, a risk-based approach where decisions are grounded in evidence and site-specific assessment rather than template compliance.
That transition has produced growing quantified evidence of what happens when prescriptive TTM gives way to risk-based TTM. The results are consistent across project types, and they were not what most people expected.
The sweet spot
Every work zone has an optimal level of traffic management. Below it, people are under-protected. Above it, the TTM itself starts creating new hazards.
A welfare-economic model developed by researchers provides the framework. The model identifies an optimal point where the last dollar spent on TTM reduces expected harm by exactly one dollar. Below this point, workers and road users lack adequate protection. Above it, additional TTM begins creating new risks that partly offset the safety it provides.
Illustrative model based on the welfare-economic framework. Actual curves vary by site context, traffic volume, and work type.
Why can more TTM create risk? Because every cone placed on a live road requires a person to place it — and retrieve it — on foot, in live traffic. Every additional sign increases visual clutter and can reduce driver comprehension. Every expanded layout requires more frequent maintenance visits. Every maintenance visit puts crews on live carriageways.
The evidence
Six case studies across distinct project environments. Click any card to see what the data shows.
The controlled trial
A 14-stage residential undergrounding project provided the setting for a controlled comparison. Stages 1–7 operated under prescriptive TTM. Stages 8–10 switched to risk-based TTM on the same road, same contractor, same conditions.
The dataset: 226,000 vehicle records, 1,820 hours of AI-analysed video, 424 resident surveys, and 99 detailed on-site records over 46 normalised working days.
Risk events (time-to-collision under two seconds) fell 15.4%. High-severity events fell 20%. Speed through the work zone decreased 12.6% under risk-based versus 4.1% under prescriptive — three times more effective. Staff hours fell 17.5%. Vehicle hours fell 45.5%. At maturity, the analysis projects 15–20% total on-road cost reductions.
Cost reductions vary by project type. On low-impact works (inspections, maintenance), risk-based TTM can eliminate the TTM cost entirely for activities where the prescriptive framework required a full setup that the actual risk profile did not warrant. On complex projects, the savings are more moderate but still consistent.
Cost reductions across six project environments. The water services and urban CBD figures represent specific documented scenarios, not universal rates. Controlled trial data tested at p<0.05.
The controlled trial measured safety directly using AI-based video analysis of 226,000 vehicle movements. Risk events (TTC under two seconds) fell 15.4%. High-severity events (TTC under 0.5 seconds) fell 20%.
The Nilsson Power Model calculation showed the risk-based approach reduced fatal crash probability by a factor of four. Speed through the work zone decreased 12.6% under risk-based TTM, versus 4.1% under prescriptive — three times more effective at reducing speed.
In the urban study, 21 of 22 assessed risk categories scored lower. Workers spent 27.6 fewer hours in live-edge exposure over 30 days — 3.5 full shifts not spent adjacent to moving traffic.
On the highway bridge, the risk assessment demonstrated that installing a temporary barrier would have increased overall site risk, not reduced it. CAS data showed zero barrier breaches in 25 years and a per-trip crash probability of 1 in 2.25 million.
Controlled residential trial. Risk index derived from the Nilsson Power Model: (Vttm/Vbase)⁴ × (Qttm/Qbase).
The controlled trial measured a 33% reduction in CO₂ emissions from TTM operations. Signs fell 64%. Cones fell 27%. Public preference for the risk-based approach was 4.5 times higher: 41% preferred it, versus 9% for prescriptive.
The urban CBD study recorded 96% fewer cones, 66% fewer signs, and 40% less fencing. The work footprint was 30% smaller. An independent accessibility assessment described it as the best site of all those assessed.
Device reductions from the controlled trial (residential) and comparative study (urban CBD).
About the evidence base
The residential trial was a controlled quasi-experimental study over 46 normalised working days. Vehicle movements were recorded via pneumatic tube counters and AI video analysis. Risk events were classified using time-to-collision thresholds. The urban comparison used failure modes and effects analysis (FMEA) across 22 risk categories, with 30-day normalised periods. The highway bridge assessment used 25 years of crash analysis data and structural engineering review. The highway construction assessment evaluated 47 risks across two methodologies using a comparative framework aligned with ISO 31000:2018. The event and water services data come from documented project cost records.
See the difference
Toggle between a prescriptive and risk-based layout for the same work zone. Same road, same work — different approach.
Live-edge exposure: where the real risk sits
Every minute a worker spends on a live carriageway is exposure to moving traffic. Prescriptive TTM requires more devices, which means more time placing, checking, and retrieving them.
Real projects, real numbers
Six distinct environments. Consistent results. Click any card for the full story.
The controlled trial
ResidentialA 14-stage residential undergrounding project provided the setting for a controlled comparison. Stages 1–7 operated under prescriptive TTM. Stages 8–10 switched to risk-based TTM on the same road, same contractor, same conditions.
The dataset: 226,000 vehicle records, 1,820 hours of video, 424 resident surveys, and 99 detailed on-site records over 46 normalised working days.
At maturity, the analysis projects 15–20% total on-road cost reductions. The trial also found that on 80% of days, TTM crews waited an hour after setup before the contractor arrived — an operational inefficiency that risk-based scheduling can address.
The city comparison
Urban CBDA high-voltage electrical pit upgrade in a busy shared-space CBD street was completed under prescriptive TTM in 2022, then repeated on adjacent pits using risk-based TTM in 2025. Same street, same work type, same contractor.
The prescriptive site logged records such as "site running as per TMP." The risk-based site logged ten times more dynamic control actions — real-time adjustments to layout, pedestrian flow, and traffic interaction. An independent accessibility assessment rated the risk-based site as the best of all sites assessed.
The barrier that wasn't
Highway bridgeA multi-month construction project on a state highway bridge carried approximately 18,000 vehicles per day. The lead contractor wanted a temporary barrier installed on the bridge deck. The risk-based assessment asked a different question: would a barrier actually reduce overall risk?
The answer was no. Structural analysis showed neither of the two available barrier systems was compatible with the 152 mm bridge deck. Both required 250 mm embedment. Installation would have reduced the carriageway below safe minimums, pushed cyclists into live lanes, and increased the probability of centreline crossings and head-on collisions.
25 years of CAS data showed zero barrier breaches at the site — a per-trip crash probability of 1 in 2.25 million.
The road controlling authority ultimately instructed the lead contractor that a barrier would not be permitted. The risk assessment documented that the adopted controls addressed all identified risks to both road users and workers.
The construction comparison
Highway constructionA road construction programme compared two methodologies — single-lane alternating flow (automated traffic signals) versus two-lane diversion with barriers — across a formal risk assessment of 47 discrete hazards using fault tree analysis.
The conventional approach required a temporary barrier system, a constructed diversion, and higher ongoing TTM staffing. The risk-based option used automated signals with minimal physical infrastructure. On a 600 AADT road, the barrier system alone would typically cost NZ $200,000–$500,000 over a 12-month construction period, compared with NZ $60,000–$80,000 for portable traffic signals.
The assessment found that the barrier system did not improve the overall risk profile. It documented how additional controls introduced their own risks — the dynamic the welfare-economic model predicts.
The event build
Event / UrbanA 58-day event build and pack-in required road management in an urban, low-speed, low-volume environment. Under the traditional approach, the TTM package included staffed road closures, overnight site checks, and daily equipment hire.
Overnight site check: 58 × NZ $192 = NZ $11,136
Daily TTM equipment hire: 58 × NZ $120 = NZ $6,960
Boom gate delivery/removal: 2 × NZ $140 = NZ $280
Install & removal TTM: 8 × NZ $118 = NZ $944
Custom training for security: 3 × NZ $250 = NZ $750
The risk-based approach replaced permanent staffing with automated boom gates and trained security personnel. The 93% cost reduction did not come at the expense of safety — the boom gate system provided continuous physical access control that staffed closures could not match during shift changes and breaks.
The invisible cost
MaintenanceA water services operator needed to inspect assets on road shoulders twice daily. Under CoPTTM, each inspection required a shoulder closure with full TTM setup — signs, cones, advance warning — at a cost of NZ $340 per visit.
Under the NZGTTM, a pre-assessed practice note diagram covers exactly this scenario. The inspector parks on the shoulder, operates within the safe zone, and no TTM hire is required.
NZ $3,400/week × 52 weeks = NZ $176,800/year
No TTM hire required for this activity.
This is one team on one activity type. The saving reflects a documented scenario where the prescriptive framework required a TTM setup that the actual risk profile did not warrant. Water services operators across New Zealand run hundreds of similar inspections daily. Not every inspection will see the same result, but for low-risk shoulder work where the operator does not interact with live traffic, the NZGTTM practice notes remove the need for TTM entirely.
Where the ROI sits
The return on risk-based TTM depends on the type of work. Lower-complexity jobs show the largest savings because prescriptive frameworks over-specify them the most.
Your project ROI
Estimate the return from risk-based TTM on your own project. Select your work type, adjust the inputs, and the model applies evidence-based savings rates drawn from the case studies above.
Calculator methodology
Savings rates are tiered by work type and maturity level, drawn from documented evidence. Non-invasive maintenance: 70/85/95% (early/standard/optimised) based on the water services case where prescriptive TTM was eliminated for a specific activity type. Event management: 60/80/90% based on the 93% reduction observed in the event build case. Urban construction: 5/15/25% based on the controlled trial (4.9% at early adoption, 15–20% projected at maturity). Highway works: 5/12/18% based on the controlled trial evidence scaled for highway contexts. Complex construction: 5/10/15% reflecting the more variable nature of savings on multi-stage projects. The control avoidance toggle adds a one-off saving for scenarios where a risk assessment eliminates a high-cost control that would otherwise have been installed. Live-edge exposure is estimated at the work type's exposure factor multiplied by crew size and duration. CO₂ uses the 33% reduction observed in the trial, scaled by project intensity. These are estimates. Actual results depend on site conditions, crew competency, and implementation quality.
What this means
The implications differ depending on where you sit in the industry.
For asset owners
Risk-based TTM is not a cost-cutting exercise. It is a duty-of-care exercise that also happens to cost less. Under the Health and Safety at Work Act 2015, duty holders must eliminate or minimise risks so far as is reasonably practicable. Over-specification is not conservative — it transfers risk from one category to another. The evidence now quantifies that transfer.
For contractors
The transition requires investment in competency, technology, and culture. The evidence shows the return is substantial: lower operating costs, fewer safety events, shorter programmes, and better community relationships. The controlled trial also found that crews not yet operating to the efficiency the designs enable is the primary barrier to full financial return.
For the industry
New Zealand is shifting to a progressive, risk-based TTM framework. The NZGTTM enables decisions based on evidence and site-specific assessment. The case for this transition is now supported by controlled trial data, comparative studies, and project cost records across six distinct environments.
For road users
Smaller, clearer work zones. Less visual clutter. More effective speed management. Less disruption to daily travel. And a 4.5-to-1 public preference for risk-based layouts over prescriptive ones. What is better for workers is also better for the public.