Current anomalies in guidelines for setting 20mph speed limits in Scotland

Note this was supersceded in 2015 - See our new Press Release

England, Wales, Scotland and Northern Ireland all have guidance on setting speed limits which is based on DfT Circular 01/2006[1]. This replaced the previous guidance within DfT Circular 01/1993. The 2006 guidance change was seen by local authorities in England as a basis for them adopting a more community-aware approach to implementing mandatory 20mph limits as the default from residential streets without traffic calming. To date this has been implemented or committed to by authorities in Portsmouth, Oxford, Warrington, Liverpool, Islington, York, Lancashire, Oxford, Bristol, Brighton & Hove, Camden and others.

In Scotland the 2001 guidance had suggested the use of advisory 20mph limits where previous 85^th percentile speeds were above 24mph and no traffic calming was to be used. This was incorporated within previous 2001 guidance for Scotland only[2]. In such streets the mandatory limit remains at 30mph so the advisory limit has no legal status. When the Scottish government issued its revised Speed Limit Guidance (ETLLD Circular 1/2006[3] )based on the 2006 DfT document it omitted 20mph limits from this guidance change and referenced the 2001 guidance.

This meant that whilst the guidance developed in 2006 (by DfT) advocated the use of  mean speed to assess the appropriateness of 20mph speed limits without traffic calming, the 2001 Scottish guidelines were using 85^th percentile speeds (usually higher) and consequently guided local authorities to advisory rather than mandatory 20mph limits.

Since 2006 the DfT have issued a further update to its 2006 guidelines which recognised the benefits of 20mph limits in places where previous mean speeds had been above 24mph. A 7mph average reduction was quoted. It also referenced the benefit to be derived from authority-wide 20mph default mandatory speed limits for residential roads.

Note that all of the above applies to 20mph “limits” rather than “zones”. The latter do not need repeater signs, but do need repeating traffic calming devices (usually physical). “Zones” have been about 50 times more expensive than “limits”. Hence for wide area interventions across complete authorities then limits are preferred for many reasons including consistency, maximising benefits and cost effectiveness..

The issue has now been further complicated (and made more flexible) by the latest TSRGD (Signage regulations) classing 20mph repeater signs and carriageway roundels as traffic calming devices. This effectively means that “zones” can be implemented using a mixture of repeating signs, roundels and other traffic calming devices. Hence for most practical purposes there will be little difference between a zone and a limit in terms of the ability to use non-physical measures.. Cheaper, signage based 20mph “zones” can be widely deployed to meet both DfT and Scottish guidance

The current status is that in Scotland the guidance on 20mph limits is different from that in England. It is thought that this difference explains the far greater use of 20mph limits in English towns compared to Scotland. However this need not bar Scottish Traffic Authorities from taking note of the guidance and making their own judgements of the correct speed limit for their streets. Indeed it is the local Traffic Authority that is responsible for speed limits rather than the national government (except for Trunk roads).

The contradiction in Scotland is that the principles and objectives incorporated within the 2006 guidance on setting speed limits generally are not referenced within the previous 2001 guidance to be used for 20mph limits. Hence whilst the 2006 guidance may require that :-

“28. The underlying aim of speed management policies should be to achieve a 'safe' distribution of speeds which reflects the function of the road and the impacts on the local community.”

And

“32. Different road users perceive risks and appropriate speeds differently. Drivers and riders of motor vehicles often do not have the same perception of the hazards of speed as do pedestrians, cyclists and horse riders. The needs of vulnerable road users must be fully taken into account in order to encourage these modes of travel and improve their safety. Setting appropriate speed limits is a particularly important element in urban safety management, with significant benefits for pedestrians and cyclists.”

In addition whilst the 2006 guidance says

35. Mean speeds should be used to determine local speed limits as this reflects what the majority of drivers perceive as an appropriate speed to be driven on the road. It is also felt to be easier for road users themselves to understand. This is a change from the use of 85 ^th percentile speed advised in Circular 1/93, which refers to the speed at, or below which 85 per cent of the traffic is travelling.

The 2001 20mph guidance does not include these and specifically uses 85^th percentile rather than mean speeds.

We therefore believe that there is room for both the clarification of the guidance on setting 20mph limits to incorporate the principles and objectives of the 2006 general speed limit setting guidance, and also to update it to take account of the considerable success of 20mph limits when implemented across complete traffic authorities as a default. This should also take account of the forthcoming change to the TSRGD which will enable more flexibility in the use of 20mph zones without the specific need for physical calming.

More information on the use of 20mph limits by Traffic Authorities can be found on the 20’s Plenty for Us website.  In particular the following “briefing sheets” may be useful.

• 20mph limits are 7 times more effective than isolated 20mph zones[4]
• 10 ways that 20mph limits benefit drivers[5]
• How 20mph limits benefit the police[6]
• 20mph limits improve air quality[7]
• 20mph Limits encourage walking and cycling[8]
• How school safety zones are not a priority![9]