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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Turkish delight

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

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UK design guideline TD 9/93 – time for a rethink ?

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

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Parameters (3) vehicle type

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

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Parameters (2) road type

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

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Criticism of current standards

The authors Broeren, Uittenbogerd, Groot, Ruijs gave a paper at the ISHGD 2015 (5th International symposium on highway geometric design). The paper has the title “Insight in horizontal curves – fundamental research on behalf of the update of the Dutch road design guidelines”. Even before they get into their detailed discussion the authors make some interesting comments, for example: Continue reading

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Parameters (1) – fundamental parameters

I discussed the idea of many parameters in highway geometrics, in other blog posts, for example here and here.

There are many different parameters involved in the design of a highway. So far I have a list of more than 230, and I am still adding to it. Some parameters are related to and perhaps derived from others – so for example, differential speed is related to if not also derived from design speed.

If we can use the idea of “relationships” to give a structure to the many design parameters, it may be possible to get a better understanding of what we are doing. In theory we could arrange the different parameters into several layers of detail (1st level parameters, 2nd level parameters, 3rd …, 4th …. etc). But the first question is to state what the fundamental parameters are. At the present time I would suggest there are seven (possibly eight) :
1 Road type
2 Vehicle type
3 Road users
4 Geometrics
5 Geography
6 Economics
7 Engineering
8 Aesthetics (?)

Maybe brief notes on each of these parameters could take the discussion further.

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Road design standards

A new version of the “Road design standards” document has just been made available. It covers 79 countries, 7 multi-country standards and 3 special topics. For more details visit the Road Design Standards website.

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Ireland and document libraries

Ireland’s “Transport Infrastructure Ireland (TII) “is responsible for managing and improving the country’s national road and light rail networks”. TII has just (June 2016) launched a publications website, which offers access to documents such as standard cross-sections.

The country’s Department of Transport, Tourism  and Sport (DTTAS) has its own publications website, whilst the National Transport Authority (NTA) also makes available documents for download. Maybe these three sources could be merged into a single, searchable library sometime.

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