20mph limits are 7 times more effective than isolated 20mph zones

Explains the cost comparisons between 20mph limits and 20mph zones with physical calming to show that limits are far more effective per pound.

Pound for pound spent, 20 mph limits with signage are 7 times more cost effective than 20 mph zones with humps.

20mph speed zones with traffic calming are undoubtedly effective and in London have resulted in an average 9 mph reduction in speed[1].

Recent evidence from Portsmouth shows that, when implemented on a wide scale without traffic calming, (what we call “Total 20”), 20 mph speed limits reduce casualties by 22% and have a varied speed reduction effect dependent upon previous average vehicle speeds. Roads with pre-existing low speeds reduced less, whilst roads with higher speeds reduced by an average of 7 mph. The average speed reduction for 20 mph limits across the whole network was 1.3 mph[2]. Bristol’s Bedminster and Southville pilot 20 mph limit led to a 1.4 mph average speed reduction[3].

Portsmouth spent just £1,100 per km for limits compared to £60,000 per km for physically calmed zones. Comparing £100,000 spent within a community with 50 miles of roads we find that 20 mph limits with signage alone gives much better value for money than 20 mph zones.

Option 1 - Spending £100,000 on 20mph zones with physical calming.

This funds just one mile of streets with a 20 mph zone with physical calming. Average speed drops by 9 mph. As the speed limit on the other 49 miles of roads remains the same then the average speed reduction across the whole network is just 0.18 mph. Note that isolated schemes endorse driving faster outside of the zone.

Option 2 - Spending £100,000 on 20mph limits without physical calming.

This will fund 56 miles of streets with a 20mph limit. Hence it will cover the whole community. The average speed reduction will be 1.3 mph.

For any given expenditure 20mph area-wide limits are 7.2 times more effective than physically calmed zones.

When coupled with the benefits from speed reduction in terms of reduced collisions, casualties, noise emissions and danger then the use of “Total 20” to implement a 20mph speed limit on all residential roads across a community has clear financial benefits. In Warrington officers reported an 800% First Year Rate of Return when comparing the savings on casualties with the cost of implementing their 20 mph scheme. Of course subsequent years are free from cost.

[1] Webster D, Layfield R. Review of 20 mph zones in London boroughs. Transport for London, 2003 (project report PPR243).

[2] Department for Transport – Interim Evaluation of the Implementation of 20mph Speed Limits in Portsmouth.

[3] Reported in Bristol Evening Post 24 Nov 2010,  20 Mile Speed Limit For City.

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