There was a "very slight" increase in journey times on Southampton's main traffic corridors after the rollout of 20mph speed limits in the city, according to a council report.

Southampton City Council has published its phase one monitoring and evaluation report which provided an immediate assessment of the impact of 20mph speed limit schemes.

A second phase evaluation will be carried out in the future looking at the wider outcomes, such as road safety, switch of transport mode, perceptions and attitudes.

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Current council policy is for 20mph speed limits to be introduced in areas of the city in a phased approach.

Monitoring for the phase one analysis focused on the schemes Bassett and Flowers, St Denys, Polygon, and Shirley and Freemantle, which were brought in between July 2022 and September 2023.

Across 63 roads that had switched from 30mph limits to 20mph limits, the average speed had fallen from 22.92mph to 20.77mph, however, there was a variance to the impact.

“The lesson learned from the initial analysis is that streets changed to 20mph see a reduction in speeds, and the reduction is greater on streets with traffic calming installed at the same time,” the report said.

“This gives confidence that the 20mph programme is achieving the policy goal.”

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Data from Hill Lane and Shirley Road found journey times had “very slightly increased” – seven to 14 seconds on average for Hill Lane and by around six seconds for Shirley Road.

“This indicates that speed is not a factor in the impact on journey times but other factors on the corridor, such as turning traffic or traffic lights are a larger impact on journey times,” the report said.

It was too early to conclude whether the speed limit changes had reduced casualty numbers from crashes, the analysis added.

The report said there had been vandalism in Shirley and Woolston.

This typically took the form of 20mph signage being sprayed out, which was similar to incidents in Wales after a national roll out was introduced.

For future schemes, the council plans to install higher signage, use anti-graffiti material signage and make more use of road markings instead of repeat signs in a 20mph stretch of road.

Summing up the lessons learned, the document said: “The introduction of 20mph represents a major change in Southampton and the way people travel, live and work in the city.

“The new lower speed limits rely on a shift in driver behaviour which takes time to become normal.

“This initial analysis shows that in the areas where the new limit has been implemented there has been broad compliance, but environmental and behavioural factors influence the level of compliance and public perceptions.”

The report said the council intends to continue the rollout of its 20mph speed limit programme, subject to funding, with plans in the city centre, Fairisle and Peartree areas.

In late 2024, the local authority carried out a consultation on reverting sections of Hill Lane, Paynes Road, Romsey Road and Shirley Road back to 30mph following a review of data and discussions with the Labour administration.