Calming the streets

Angela Gill, Team Manager Traffic Engineering for Portsmouth City Council, discusses Portsmouth’s city-wide 20mph scheme for residential roads

There are two ways in which 20 mph can be installed in an area. The original way in which this was achieved was by creating a 20mph Zone. Now changes to legislation permit a 20mph speed limit to be made, as long as it can be evidenced that the existing speed of traffic is sufficiently low, without physical, speed constraining measures.
    
The requirements for introducing a 20mph zone are that the speeds within the zone are self-enforcing. This generally implies that the limit is enforced by installing traffic calming treatments such as speed humps and other traffic calming techniques.
    
However, if the existing, average traffic speeds are already low (20 per cent of the desired posted speed limit, therefore 24mph or less), then it is permitted, under guidance set out in DfT Circular 01/2006, Setting Local Speed Limits, to impose a 20mph speed limit on roads within an area. Once in place the limit is self-enforcing by way of terminal signs reinforced by a series of repeater signs, erected on lamp columns, throughout the included roads.

City layout
The layout of Portsmouth’s residential roads lend themselves to the latter treatment, in that the city has three strategic entry and exit routes and a number of primary roads interlinked by a network of distributor roads, both primary and secondary, many of which pass through residential areas. A great number of the city’s residential streets form a closely packed network, developed during the 18th Century or earlier. These roads are demonstrably unsuitable for high-speed traffic.
    
The housing is typically terraced, which in many cases give straight out onto the highway. Generally there is little or no off street parking and as a consequence the roads are heavily parked. Due to parking congestion the available carriageway space is often narrowed to a point where the roads operate as one way and often standoff situations arise, when drivers fail to give way to each other.

Qualifying areas
Prior to adopting the current policy, Portsmouth had carried out a review of the city and developed a priority list of ten potential 20mph zones. There was an agreement to implement two zones per year over a five year period at a cost of approximately £200,000 per zone and £2 million to complete the programme.
    
Construction of the first zones was begun while at the same time a triple fatality led to the need to react swiftly to public demand for the speeds on one of the city’s routes to be lowered. An experimental scheme was installed and at the same time measurements of the speed and volume of traffic on the surrounding roads was carried out.
    
From these surveys it was apparent that the existing speeds on the surrounding roads were sufficiently low for them to be included in the order. In this way the area wide lowering of the speed limit to 20mph was born. At the same time a Safer Routes to School scheme was being consulted upon, the results of which also indicated a public wish for a 20mph speed limit both outside the school and also in the surrounding estate roads.
    
The success of these two schemes subsequently led to the decision to expand the lowered speed limit to all the residential roads throughout the city. In this way it would be clear to the travelling public, when they left the main road network and entered side or residential roads, that the limit would be automatically 20mph indicated by terminal signs and repeaters at intervals along the included roads.
    
The city was divided into six residential sectors for consultation and construction ease. The total cost of the scheme is in the region of £600,000 and early indications show widespread public satisfaction with the new speed limit.

One year on
The scheme in its entirety has now passed its first year anniversary and post scheme monitoring is currently being carried out, with an interim evaluation document to be published in the late summer by the DfT.
    
Although the evaluation is still to be completed, it is believed that the 20mph benefits include:

• Improved road safety especially for pedestrians and cyclist
• Civilised traffic and improved environment
• Reduced noise pollution
• Neutral impact on air quality

The vision for developing this approach was and is, to ensure the city’s residents can enjoy a safer and healthier environment and that the streets outside their houses revert to community areas as opposed to hostile traffic corridors.
    
“The Safer Roads Partnership has been working closely with Portsmouth City Council to ensure that the new 20 mph zones are signed in accordance with all regulations and guidance,” comments the Safer Roads Partnership.

“Monitoring of speeds in the new limits will be carried out on a regular basis, and will assist in identifying the level of compliance with the new limits. The Partnership is currently working with the Council to develop a range of initiatives designed to improve compliance at sites where this may be needed. That could include enforcement as well as education initiatives, but details have yet to be confirmed.”