Sight distance in motion

In an earlier post I listed something like 44 different types of sight distance (see here). Even this list is not complete. For example I recently came across the term “sight distance in motion” (SDM), a parameter used in reference to the design of facilities for cyclists. One source (ref. 1993) says that SDM :

“…. could be called the comfort visibility zone when cycling. It is the distance that a cyclist needs to see ahead in order to make riding feel safe and comfortable. Research has determined this to be the distance a cyclist covers in 8 to 10 seconds i.e. between 50m and 80m at typical cycling speeds”.

Another source (ref. 745) says more or less the same, and refers to research from CROW in 1993, whilst a third (ref. 1937) also quotes the values of 50m and 80m for SDM.

(Ref.1993) also says that “SSD is the distance that a cyclist needs to see a hazard, react to it and come to a halt. This is shorter than the SDM”. For example, (Ref. 1937) quotes a recommends 25 m for SSD at 30 km/hr and 80m for SDM.

(Ref. 1993) gives a range of values for SDM, which are shown in the following table.

SDM 1

Comment

I am not sure why a cyclist needs three times the SSD to feel safe and comfortable (or even how “safe and comfortable” is measured. There is also the question that, if SDM is a valid parameter in the design of routes for cyclists, why is it not also a valid parameter in the design of routes for cars and trucks (etc.).

References

745 – UK, LTN 2-08 Cycle infrastructure design; Department for transport, October 2008

1937 – UK, Cardiff cycle design guide, Cardiff cycle network, 2011

1993 – UK, Design portfolio B.04 junction and forward visibility; cycling england, 2008

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About roadnotes

Robert Bartlett is an international consultant with over 30 years of professional experience as a highway and traffic engineer with leading companies and organisations in several countries, including Germany, China (Hong Kong), Qatar and the UK. Specialised in urban studies, transport and the use of GIS, research has included new ideas on subjects such as the study of social justice using GIS, the dimensions of vehicles, and comparative geometrics (highways and transport).
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