Engineers might think that the posted speed limit is based either on the design speed or on a measure such as the 85th percentile speed of traffic on an existing road,, and that theposted speed “is a speed limitation …. aimed at encouraging drivers to travel at appropriate speeds for all prevailing conditions” (Ref.148). Neither of these suppositions is necessarily true.
For example, posted speed limits can vary with
The time of day – there is at least one section of major road in Germany where lower speed limits apply at night ( to help reduce traffic noise in residential areas)
The structural condition of the road surface – lower speeds can be posted for sections of road with a damaged surface
Political pressure – either to increase the posted speed limit, as in the UK in 2011:
“The Department of Transport is to launch a consultation on increasing the speed limit on England and Wales’ motorways from 70mph to 80mph.Transport Secretary Philip Hammond said the current limit, introduced in 1965, was out of date due to “huge advances in safety and motoring technology” (from the BBC website here).
Or to reduce the posted speed limit, as in the UK in 2015:
“Speed limits could be lowered to 60mph on sections of several British motorways in moves to meet European Union rules on fighting dangerous levels of air pollution. The move marks a dramatic change of policy just two years after the Government was advocating an increase from 70mph to 80mph on major routes to shorten journeys and boost the economy”. (From the Independent website, see here).
At the same time, posted speed limits do not vary with
With reduced visibility – such as at night or in fog
With poor road surface states – such as in heavy rain or with icy road surfaces
(I know the posted speeds can vary with sections of road which have variable message signs, but most roads don’t have them).
And anyway, posted speeds do not apply
To all vehicle types – such as large trucks , passenger coaches etc which have type-specific speed limits.
The reasoning behind a posted speed limit may not be as consistent as engineers might think. Maybe a new definition of posted speed limits might be more on the lines of:
“Posted speed limits may indicate a safe driving speed – but only for cars driving in good visibility and weather conditions, and with little road traffic. However posted speeds may also indicate a temporary political preference and so may not be related to either safety, road geometry or design speed”.
There are also implications for the geometric design of a road. For example, do you design for trucks travelling at the design speed, or trucks travelling at what might be a changeable type-specific limit (in late 2014 the UK government announced plans to raise the speed limit for lorries travelling on dual carriageways from 50 mph to 60 mph (see here).
And if you change a posted speed limit, do you have to redesign the road so that the geometry is appropriate to the lower / higher speed?
148 – South Africa, “Geometric design guide”, CSIR; 2002
831 – USA, “A policy on the geometric design of highways and streets” AASHTO; 2011