Outside Hamilton, New Zealand, the State Highway 1 (SH1) comes to a T-intersection with the State Highway 29 (SH29) and Tirau Road. The right-hand turn from SH29 to Tirau Road caused significant congestion and long delays, resulting in a double fatality and 15 serious accidents in recent years.
In late 2024, the New Zealand Transport Agency (NZTA) replaced the T-intersection with a roundabout, aiming to reduce travel times by 50% as well as other benefits.
After installing the roundabout, NZTA used Path Analysis to measure any changes in speeds. They found that average and median speeds decreased by 2-6km/h after the intervention. Meanwhile, 85th percentile speeds increased by 4km/h.
There was also a 53.8% decrease in average travel time, from 2:23 minutes to 1:06 minutes. This new average travel time remained consistently lower throughout the day, with PM traffic experiencing fewer delays compared to before the roundabout was installed.
Before the intervention, 36% of vehicles made one or more stops when trying to turn right. After the intervention, all recorded vehicle trips made it through the roundabout without any stops. These free-flow traffic conditions helped to reduce congestion and delays, contributing to the reduced travel time through the intersection.
The new roundabout successfully improved traffic flow and lowered travel times. These insights can also inform future infrastructure planning and business case development for similar projects.