Policy instruments (3) - Transport Policy and Planning in Great Britain - page 228

Indeed, if a speed limit is set in isolation, or is unrealistically low, it is likely
to be ineffective and lead to disrespect for the limit …. This may also result in
substantial numbers of drivers continuing to travel at unacceptable speeds, thus
increasing the risk of collisions and injuries.
(DfT 2006h paras 21 and 22)
In determining appropriate speed limits the aim should therefore be to provide
a consistent message between the geometry of a road and its environment and
for changes in the limit along a route to reflect changes in these characteristics.
Where the basic physical characteristics of the highway suggest a higher speed than
is appropriate given its accident record, additional engineering and/or landscaping
measures as well as the imposition of a (lower) speed limit will be necessary. The
costs of these as well as the effects on traffic capacity and journey times will need to
be weighed against the prospective benefits. Only where the requisite engineering
measures are impracticable or unsuitable should reliance on camera enforcement be
resorted to (see below).
At present within rural areas the vast majority of the network is subject to the
national 60 mph limit for single-carriageway roads. However in many cases drivers
do not reach this speed because the characteristics of the road make it impractical
or unsuitable. Nevertheless almost a half of all serious casualties take place on these
roads and excessive traffic speed is a common complaint amongst rural communities.
Authorities are therefore encouraged to adopt a two-tier approach to local speed
limits based on whether roads have a predominant traffic flow or a local access or
recreational function (Table 14.1; also IHT 1999) and are currently required to review
the speed limits on all A and B roads in these terms by 2011 (DfT 2007k).
A hierarchical approach is also recommended for use in urban areas to ensure
that the speed and other characteristics of traffic on individual roads is appropriate
to local circumstances (IHT 2003). The standard limit of 30 mph in built-up areas
is the default limit but authorities may impose higher or lower limits in particular
places. A higher limit of 40 mph may be appropriate on higher quality main roads in
suburban areas or on the outskirts of towns where there is little development and few
vulnerable road users. Exceptionally a 50 mph limit may be appropriate for purpose-
built dual carriageways such as ring roads or major radials which have little or no
frontage development and an absence of non-motorised users.
In places where there is a significant presence of non-motorised users (e.g. shopping
streets, residential areas or in the vicinity of schools) consideration should be given
to a lower, 20 mph limit. (At an impact of 20 mph a pedestrian has a 95% chance
of surviving whereas at 30 mph it is approximately 65% and at 40 mph only 15%.)
Limits of 20 mph may either be applied to individual sections of road (except main
traffic routes) or to local areas so as to form a 20 mph 'zone' (DETR 1999a). These
lower limits are unlikely to be complied with where existing speeds are substantially
higher and in these situations it is necessary to complement the lower limit with traffic
calming measures so that the arrangement becomes self-enforcing.
Under current arrangements speed cameras cannot be used to aid the enforcement
of 20 mph limits. This coupled with the expense and unwanted side-effects of the
traffic calming measures needed to create 20 mph zones has led to a revival of debate
over whether 20 mph rather than 30 mph should be adopted as the default limit in
most residential streets (LTT 480).