Speed – and in particular design speed – is a key parameter in determining values for a number of highway geometric features in highway design. Examples include horizontal radius and stopping sight distance. There also seems to be a relationship between design speed and other measures of speed.
These include, in particular:
- Operating speed
- Posted speed
- Design speed
- 85th percentile speed
- Target speed
- Political speed
We can find notes on these other speed parameters (except perhaps the last one) in many design standards and guidelines. The following text gives a few examples of these notes, and then suggests some conclusions which might be drawn from them.
In its paragraph 2.2.4, Austroads (ref. 1887) seems to say that operating speed is identical with design speed. However later on the document says:
“The term Operating Speed in this guide refers to the 85th percentile speed of cars at a time when traffic volumes are low, and drivers are free to choose the speed at which they travel”.
And, on design speed:
“A speed fixed for the design and correlation of those geometric features of a carriageway that influence vehicle operation. Design speed should not be less than the intended operating (85th percentile) speed“
The same reference also says that
“Wherever possible, operating speeds should be measured for both cars and trucks, in both directions of travel”
This all seem to suggest that
- The 85th percentile speed is only for cars
- A road can have several operating speeds (by direction of travel and type of vehicle)
From Austroads, (again, ref. 1887) we have:
“Where the operating speed cannot be determined through speed measurement or by the Operating Speed Model, designers shall adopt an operating speed 10 km/h higher than the legal (posted) speed limit”.
The USA’s AASHTO Green Book 2011 (ref. 831) says:
“Posted speed limits, as a matter of policy, are not the highest speeds that might be used by drivers. Instead, such limits are usually set to approximate the 85th percentile speed of traffic as determined by measuring the speeds of a sizable sample of vehicles”
From a document from South Africa (ref. 148) we have:
“Various studies have shown that the 85th percentile speed generally exceeds the posted speed limit by a margin of at least 10 km/hr when weather and traffic conditions are favourable.”
Taken together, we have what seem to be conflicting ideas
- Posted speed should be 10 km/hr less than operating speed
- Posted speed should be the 85%ile speed
- Posted speed is less than the 85th percentile speed
From South Africa (ref. 148) we have:
“Various studies have shown that the 85th percentile speed generally exceeds the posted speed limit by a margin of at least 10 km/hr when weather and traffic conditions are favourable. For this reason, design speed is typically equated to the 85th percentile speed“
However (Ref. 2360) says simply that design speed is:
“A speed selected to establish specific minimum geometric design elements for a particular section of highway or bike path”
Similarly a source from the USA (ref. 2327) says
“Design speed is a selected speed used to determine the various geometric design features of the roadway”
Keith Wolhuter (ref. 2247) expresses it even more briefly, when he says:
“In short, design speed is the speed selected for design! “
(Ref. 1036) says that
“Design values of the side-friction factor vary with design speed. Design values represent wet pavements and tires in reasonable but not top condition”
This suggests that design speed also applies to wet pavements.
The Illinois DBE Manual (ref. 2327) says
“The 85th-percentile speed is the speed below which 85 percent of vehicles travel on a given highway. The most common application of the value is its use as one of the factors for determining the posted, legal speed limit of a highway section. In most cases, field measurements for the 85th-percentile speed will be conducted during off-peak hours when drivers are free to select their desired speed”
With the implication being that “vehicles” equals both cars and trucks. However, arguably cars travel faster than trucks, so that in real terms the 85%ile speed could well be the 85%ile speed for cars alone. This argument is reinforced by the fact that some types of heavy vehicle have their own speed limits, regardless of any potential free flow speed.
And fom the UK (ref. 1038):
“The 85th%ile free speed is generally regarded as the most appropriate choice for design speed; to design for the 99th percentile speed would be excessively expensive while use of a 50th percentile value would be unsafe for the fastest drivers”
This implies that roads designed for the 85%ile speed could still “be unsafe for the fastest drivers”, that there will be accidents, but the extra cost of the road would be more than the cost of the accidents.
I assume that because speeds are lower at night and in other conditions of poor visibility, the 85%ile speed is measured during the daytime.
- From the USA (ref. 1013) – “The target speed is usually the posted speed limit”
- (Ref. 1252), also from the USA, says that “Target speed is the desired speed of the 85th percentile vehicle”
In an earlier blog post I wrote that sometimes political pressure can be exerted, 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).
I therefore suggested that “posted speeds may also indicate a temporary political preference and so may not be related to either safety, road geometry or design speed” ”
We can reach some conflicting conclusions from the above notes:
• Operating speed (design speed) is the 85%ile speed of cars
• Posted speed = 85% of maximum speed ,
• The target speed is usually the posted speed limit
• Target speed is the desired speed of the 85th percentile vehicle
• Design speed = operating speed = posted speed = 85%ile speed = target speed
• Design speed = posted speed + 10 km/hr, and
• Design speed ≥ operating speed
• Design speed is greater than the operating speed and posted speed
Perhaps local authorities should not fine drivers for exceeding posted speeds by 10 km/hr or by 15%, because the roads are specifically designed to allow for this (speeding in these cases is the fault of the road designer, not the driver)
• Design speed must relate to wet roads, in daytime with good visibility
• Design speeds should be different for cars and trucks (and by extension, for other types of road traffic)
• Design speed varies with direction of travel
A section of road may have many design speeds, in particular for
• Each direction of travel
• Road when dry, road when wet
• Different types of vehicle
• Daytime and night-time driving
If posted speeds (usually) apply to all types of vehicle then they may be too high for safety, for truck drivers
If posted speed limits are a political rather than a technical value, and can be changed even after a road has been built, then – in engineering terms – they may not have any meaning at all.
There is a inconsistency and vagueness in the many design guides when it comes to measures of speed. All the quotations above come from English-language documents. There would probably be even more confusing results if they were to include comments from documents in other languages as well. A statement from a document (ref. 1573) published by the USA’s TRB seems to confirm this:
“Desirably there would be strong relationships between design speed, operating speed, and posted speed limit and these relationships could be used to design and build roads that would produce the speed desired for a facility. While a relationship between operating speed and posted speed limit can be defined, a relationship of design speed to either operating speed or posted speed cannot be defined with the same level of confidence”.
Even the concept of design speed as a useful tool is itself in doubt, with discussions looking at ideas such as “context sensitive roads”. Here for example (ref. 802) refers to research by Garrick and Wang (2005) which” examined context-based alternatives to the use of design speed as a controlling criterion for design of streets and highways”.
The weakness of current theory and application of speed concepts must have a bad effect on safety and accident rates. It might even be argued that the people who produce and use design guidelines are responsible (and perhaps liable) for at least some of the road traffic accidents which occur today.
- 148 – South Africa, Geometric design guidelines, CSIR 2002
- 802 – USA, NCHRP Synthesis 432 – recent roadway geometric design research, TRB 2012
- 831 – USA, Policy on the geometric design of highways and streets AASHTO 2011
- 1013 – USA, New York City, Street design manual, New York City DOT 2009
- 1036 – USA, Roess et al, Traffic engineering, 3rd edition, Pearson Education International 2004
- 1038 – C.A. O’Flaherty and others, Transport planning and traffic engineering, Elsevier 2006
- 1252 – USA, Reid Ewing and Michael King, Flexible design of New Jersey’s main streets, New Jersey DOT 2002
- 1573 – USA, Design Speed, Operating Speed, and Posted Speed Limit Practices, TRB 2003 Annual Meeting
- 1887 – Australia, AGRD part 3: Geometric design, Austroads 2010
- 2247 – Keithh Wolhuter, Geometric design of roads handbook, CRC Press 2015
- 2327 – USA, Illinois BDE Manual, Illinois DOT 2016
- 2360 – USA, California highway design manual, California DOT 2016
- 2362 – Australia, Queensland RPDM 1st edition / chapter 6 – speed parameters, Queensland department of main roads, 2007