Design standards

A textbook on highway engineering once said that there are three topics to consider in road design: the vehicle, the user and the road. We can add two more to this list:

  • Road standards – There are very many of these. They may contradict each other, be out of date or simply wrong
  • The person designing the road – who may misread the road standards, or make human errors when preparing the design

So for example, any investigation of a road traffic accident should include a look at the design of the road, the road standards used in preparing it, and perhaps the person / or organisation which designed it.

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More notes on road design standards (1)

Many countries publish “standards“, notes, advice and requirements related to the geometric design of roads. But as I have argued before (see for example here) there are too many of these standards. What makes life more difficult for engineers is that most of these standards suffer from one or more of the following problems:

  1. Traceability

A standard may suggest values for a parameter such as “side friction” – but does not say where the values come from. For all the reader may know, they may be based on research made in another country almost a century earlier. The reader should be able to trace back the suggested values to their original source.

  1. Completeness

The information presented may be incomplete. To take “side friction”  as an example again, perhaps the standard only gives values for one vehicle type, or the accompanying notes forget to mention that correction factors were applied to observed values.

  1. Terminology

The standard may not define the terms which it uses. For example, perhaps the values suggested for side friction refer to wet , surfaced roads, and are “maximum side friction factors accepted for design purposes” (or desirable maximum, or whatever).

  1. Inconsistency

Occasionally a standard may give different values for the same parameter, either internally or where the standard is part of a series of documents.

  1. Findability

Sometimes, suggested values are hidden in a dense jungle of explanatory notes, or spread through different sections of the same document; or the document may not have a table of contents. This makes it difficult to find values you are interested in.

  1. Technical

Many of the parameters involved in road geometric design can be associated with different design approaches, theories and concepts. Some of these approaches and concepts may be out of date, or doubtful, or perhaps not yet widely accepted.

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Chile’s Road design manual updated

Chile’s Roads Department, part of the Ministry of Public Works, has just updated its Manual de Carreteras. The documents can  be downloaded free of charge from (but you need to register and log in).

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60 mile/hour speed limit?

Around four years ago I wrote that, according to an article in the UK’s Daily Mail (see here), the British government had plans to increase the speed limit on motorways from 70 mile/hr to 80 mile/hr (from 112 km/hr to 128 km/hr).

Recently the same newspaper reported that the British government now has plans to reduce the speed limit on motorways from 70 mile/hour to 60 mile/hour (see here).

Not sure how these policy changes fit in with the idea of consistency in design.

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“Findability” and road design standards

I have just published some notes on “side friction”, which you can download (for example) from Academia. The document has a “discussion” section which in part looks at how current design standards fail their users. Examples of these failures include inconsistent terminology and lack of traceability.

Another one, which I plan to add to the next version of the document, is “findability“, in the sense of “is the information I am looking for, easy to find?” To take two examples, in a Tanzanian standard (ref. 294) the information is easy to find, whereas in the USA’s AASHTO document (ref.831) it is not so easy to find.

Of course, it does not follow that, even if the information is easy to find, the information is correct.

294 Tanzania, Road Geometric Design Manual (2011 ed), Ministry of Works 2011
831 USA – A policy on the geometric design of highways and streets 2011, AASHTO 2011

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Road design and older drivers

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There used to be a saying that road designs were a function of the road, the vehicle and the driver. It now appears that there are different types of driver, and some standards are appearing which suggest different values for design when the driver type is an “older driver” (ref. 2439) or a member of the “aging population” (ref.2441). It appears the USA document was first published in 1998.


Ref. 2441 USA, Handbook for designing roadways for the aging population 2014 , FHWA, 2014
Ref. 2439 Austroads, Road safety environment and design for older drivers, 2000

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ARRB resources

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The ARRB is the Australian Road Research Board. Its website says that it “exists to serve the research needs of its Members, and maintain its reputation as a national interest, applied research organisation. Hence, our vision is born: to deliver an ‘adaptable connected future’ for every road user”.

The ARRB has a Knowledge Base, which is a searchable library of documents on topics such as road and bridge design. Many of the documents can be downloaded. The service seems to work, and to be quite useful. Here is the link to the Knowledge Base:

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London – Doubtful project development procedures

On the 7th April 2017 the Independent newspaper reported on what it described as Dame Margaret Hodge’s “damning” independent review of the London Garden Bridge project. This project, as London’s official website says: “….is a proposed footbridge and public garden over the River Thames, linking Temple with the South Bank. The project is being led by the Garden Bridge Trust”.
Dame Hodge’s report (which can be downloaded from this link) draws a number of severe conclusions about the control of the project, including:

On value for money
“Decisions on the Garden Bridge were driven by electoral cycles rather than value for money. From its inception when there was confusion as to its purpose, through a weak business case that was constructed after contracts had been let and money had been spent, little regard has been had to value for money”.

On escalating costs
“The project has already used £37.4 million of public money and the agreement to underwrite cancellation costs by the Government could bring the bill to the taxpayer up to £46.4 million. I believe it is better for the taxpayer to accept the loss than to risk the additional demands if the project proceeds”.

On conduct and procedure of Transport for London and the Greater London Authority
“The procurements subject to this review comprised one contract that was awarded to Heatherwick Studio for design and consulting services and one contract that was awarded to Arup for engineering and project management services. These were not open, fair or
competitive procurements and my review revealed systemic failures and ineffective control systems at many levels”.
(In the above quotes, I have highlighted parts of the texts)

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Notes on “design speed”

designspeed2-engineering-15Speed – 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. Continue reading

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Inter-vehicle gap

The discussion on the longitudinal gap between vehicles is well-established; increased speed means increased gap. But this only covers the gap in the direction of travel. What information is there on the lateral gap – the one between vehicles at the same level travelling in adjoining traffic lanes.

From German practice

Practice in Germany sets up the idea of vehicle envelopes, and adds an amount for the space between them. The total width of these clearance spaces and vehicle envelopes gives the overall clearance envelope. The following example from (ref. 1617) shows the idea, Continue reading

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