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
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
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:
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)
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. Continue reading
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
In a recent post I expressed some doubt about the usefulness of the UK’s TD 9/93 (part of the “Design Manual for Roads and Bridges” (DMRB)). If any users of TD 9/93 also have some doubts, help may be available – it looks as if the UK government approves the use of alternative standards such as those produced by Turkey. Continue reading
The UK’s principal guideline on the design of roads is the “Design Manual for Roads and Bridges” (DMRB). Wikipedia says the DMRB “is a series of 15 volumes that provide standards, advice notes and other documents relating to the design, assessment and operation of trunk roads, including motorways in the United Kingdom, and, with some amendments, the Republic of Ireland”. Continue reading
So far I have discussed the concept of fundamental parameters in highway geometric design (for example, here) and the fundamental parameter “road type“. This post covers the fundamental parameter “vehicle type“. Vehicle type relates directly to design vehicles – the traffic which the road is to be designed for. Choice of vehicle types for a road affect such things as lane widths, cross-section and design speed(s).
On vehicle types, the 2011 edition of the USA’s AASHTO green book (ref.831) says: Continue reading
I began a discussion about fundamental parameters in highway geometric design recently. The first parameter in my list is road type. A feature of “road types” is that they are associated with specific details of highway geometric parameters in all three dimensions of horizontal alignment, vertical aligment and cross-section. A particular road type is associated with particular values of (for example) number of carriageways, sight distance, and permitted gradient. Even the name of a road type from the UK, such as a 3-lane motorway dual carriageway (ref. 1038) itself tells us much about the road cross-section, design speed and so permitted gradient.
A search of just a few highway design guidelines will easily produce over a hundred different road types, so it can help to group them into a number of sub-sets. Table 1 gives an example of how this might be done. Continue reading