If you’re a transport planner or engineer you’ll be familiar with the challenges of getting a road safety project design just right before spending money on the final product. Getting it wrong can be costly and frustrating for your community.
Many stakeholders—drivers, cyclists, pedestrians, and emergency services—have differing priorities. Therefore, it’s critical to assess safety, efficiency, and traffic flow before committing to permanent infrastructure.
Porirua City Council (PCC) faced many pressures in 2021 including a brand new motorway opening nearby and a rapidly growing population. They needed to transform a busy T-intersection and knew they had to optimise the design.
But they needed data to do so.
Opting for temporary infrastructure, including traffic separators and a roundabout, offered PCC a powerful solution for mitigating risk and optimising their final intersection design.
Let’s examine their approach.
A Success Story: The Kenepuru-Raiha Roundabout Trial
Porirua City Council faced the challenge of determining the best way to manage traffic flow and safety on local roads. This was due to population growth and the impact of future traffic from the nearby Transmission Gully motorway. As a trial, they transformed the busy Kenepuru/Raiha T-intersection in Porirua into a roundabout. Rather than immediately constructing a permanent structure, they chose to implement temporary infrastructure—a decision that proved to be remarkably successful.
The temporary asphalt roundabout and modular traffic lane separators were installed in December 2020. This trial served as an essential test ahead of the completion of Transmission Gully and its connection to Kenepuru Drive. By taking a proactive approach Porirua City Council could evaluate the roundabout’s effectiveness, and consider the number of non-standard vehicles and future traffic volumes, before committing to a permanent construction.
Why Temporary Infrastructure?
Given the numerous uncertainties in this project, opting for a temporary solution that could be modified, adapted, and monitored was a sensible choice. This approach provided several key advantages:
- Data-driven design:
Enables gathering real-world data on traffic flow patterns to identify potential bottlenecks and safety concerns before committing to a permanent design.
- Cost savings:
Upfront costs were significantly lower, allowing for adjustments based on real-world observations.
- Community feedback:
Valuable feedback from the community was gathered to ensure the final design addressed local needs.
- Flexibility and adjustment:
The temporary nature allowed for easy and immediate adjustments and modifications enabling further monitoring. It was clear from the trial that a mountable roundabout was needed for trucks to maneuver safely.
The trial’s adaptability was immediately apparent. When the roundabout was first installed, the first truck that came through knocked over an entire row of traffic separator flexi-posts. The contractors responded immediately. They lifted the rubber bases, adjusted the turning circle by a meter, and reinstalled the separator system —all within a matter of hours. This response highlights a trial project’s ability to adapt based on real-world feedback!
The Results: Fewer Crashes, Smoother Traffic Flow
The trial’s success was clear. Data collected during the operation of the temporary roundabout showed a significant decrease in crashes. In the five years preceding its installation (2015-2020), there were 19 crashes, including 12 non-injury incidents and 7 minor injuries. Since the roundabout’s installation, only four non-injury crashes have been recorded, demonstrating a strong improvement in safety and efficiency.
Darrell Statham, Manager Transport, reported smoother traffic flows, improved bus and truck maneuverability, reduced vehicle speeds, and better pedestrian and cyclist access. This compelling data directly informed the decision to proceed with a permanent roundabout.
From Temporary to Permanent: The Next Phase
After the successful trial, work started in March 2024 to replace the temporary roundabout and traffic separators with permanent, concrete features. The single lane, mountable roundabout and cycle way was complete in June 2024 and includes:
- A permanent mountable structure built from concrete, for easier truck maneuverability.
- Concrete splitter islands that cater for non-standard, large vehicles
- 3-meter-wide shared paths on Raiha St and Kenepuru Drive.
- Raised safety tables on approach to slow traffic and improve pedestrian safety.
- A designated on-road cycle facility on Kenepuru Drive.
- Landscaping and lighting improvements.
Watch the transformation here- (video credit: Porirua City Council):
Kenepuru Drive and Rahia Street Permanent Roundabout Upgrade Transition
Image: Permanent roundabout at Keneperu Drive (source: Porirua City Council Facebook 27/06/24)
PCC’s Choice: Vanguard’s Flexible Traffic Separator System and Asphalt Roundabout
There are various temporary solutions available for trial projects so you can monitor the performance of the infrastructure. It also allows you to make changes. In this case, PCC opted for Vanguard’s flexible traffic separator system and an asphalt roundabout.
You might wonder why a rubber compact roundabout wasn’t used (considering it is mountable, easy to install and remove, and provides a good turning circle) instead of semi-permanent asphalt? The short answer is, that Vanguard didn’t stock modular rubber compact roundabouts at the time. Keep them in mind if you are planning a trial project similar to this one)!
Benefits of the flexible traffic separator system
A traffic separator system encourages vehicles and cyclists to stay in their respective lanes, and away from pedestrians. Doing this makes an intersection safer for all road users (and gives peace of mind).
Vanguard’s flexible traffic separator system uses two parts: a rubber base bolted to the road that creates the physical separation and flexi-posts that insert into the bases. The latter provides vertical delineation that makes the separation easier for road users to see and follow.
They are fast and easy to install, which was important for PCC. The contractor had them on the road in one night so there was no interference during peak traffic from 6am – 6pm (keep in mind around 15,000 vehicles go through the intersection each day), and minimal disruption to nearby businesses.
They used the separator systems to create splitter islands and to create a dedicated cycle lane along Kenepuru Drive. Some residents gave feedback that it had been dangerous to cycle along this route.
Before the roundabout transformation the intersection was a classic urban T intersection with multiple turning lanes and no protection for cyclists or safe crossing points for pedestrians.
Image: Kenepuru-Rahia T intersection, before the roundabout (source: arcgis.com)
Lane separation: Trial vs permanent infrastructure
Comparing the trial infrastructure and layout with the final permanent structures show differently designed splitter islands (to allow better movement for long vehicles) and the addition of a pedestrian island on Kenepuru Drive:
 Rahaia St approach/exit. Trial: flexible traffic separators on approach to the intersection – note different layout with less deflection angle on departure lane |  Rahaia St approach/exit. Permanent: replaced by a narrower concrete splitter island with greater deflection angle on departure to reduce speed |
 Kenepuru Dr. Trial: flexible separators used for the cycle lane on the left, and splitter island on the right, keeping cyclists and drivers in their respective lanes. Also includes raised safety platform |  Kenepuru Dr. Permanent: concrete splitter islands, raised pedestrian island (not in trial) and raised safety platform (on approach) with cycle lane (green) |
A longer-than-expected trial
What was installed as a 6-12 month trial ended up staying in place for around four years until the permanent infrastructure was built. While the rubber bases stayed in place to do their job, it’s fair to say that many of the posts took a beating.
One benefit for PCC is that the posts could be easily replaced (with a twist lock mechanism into the rubber). The catch is that it did cost money and contributed to maintenance costs.
In a lower traffic environment, or for the shorter term, the system would have endured with fewer hits and fewer costs, giving better value for money, even after such a long time.
This intersection, however, was busy and carried a lot of heavy vehicles. As the trial was still in place after the Transmission Gully motorway opened in 2022, the posts were exposed to even more traffic, meaning more bumps and bangs for the posts. You can see that this type of system withstood considerable use for an extended period, but did come with a cost.
The good news is that you now have the choice of a new improved ultra-separator system that can withstand up to 100 hits at 100km/h, significantly reducing maintenance and giving you a longer-lasting solution.
Before: newly installed flexible traffic separator system
After: damaged posts along the Kenepuru Drive cycle lane. The rubber separators are still in place to encourage vehicles to stay in their lane (source: Google Maps)
Permanent cycle lane separators
When PCC installed the permanent cycle lane separators along Kenepuru Drive they chose the same type used in the trial, but with a different design. This time, the rubber sections are more spaced out, rather than the continuous layout, which allows for better drainage. This design also means less posts for vehicles to hit along the road shoulder, decreasing potential maintenance costs.
The Flexible Traffic Separator System and green painted surface clearly define the cycle lane, giving cyclists peace of mind and encouraging the separation of vehicle-cycle traffic.
Image: Rubber base with the flexible traffic separator system defining the permanent cycle lane on Kenepuru Drive
Maintaining flow: The mountable roundabout
When PCC designed the permanent roundabout, they recognized from the asphalt trial that it needed to be mountable to allow heavy vehicles to navigate the turn and maintain traffic flow. The roundabout features a concrete, mountable center island that provides an extra-wide turning circle, enabling large and long vehicles to make turns safely. Concrete is highly durable and will withstand impacts over the long term. Given that this is a busy intersection in a growing area, this design feature is critical.
 Asphalt temporary roundabout |  Concrete mountable roundabout |
Are you ready? Trial Before You Commit
Porirua City Council’s experience underscores the significant benefits of using temporary infrastructure as a crucial step in major road projects. This data-driven approach minimises risk, saves costs, incorporates community feedback, and ultimately optimises the final design for a safer and more efficient road system. For Vanguard, it added to the feedback needed to create an innovative new product to separate traffic in demanding traffic environments!
That’s a win-win for everyone.
If this looks like something you need to get your next project over the line, contact us today to learn how we can support your temporary infrastructure needs and help to make informed decisions.
