Sight distance – types of sight distance

The capacity and safety of a road substantially depend on the presence of good visibility. To ensure that the road does have good visibility the meaning of the term first must be explained and one or more ways of measuring it would have to be developed. Many highway design standards and guidelines refer to just  three or four measures of sight distance. For example the SIECA standard (ref. 1088) I believe discusses

  • Stopping sight distance
  • Decision sight distance
  • Overtaking sight distance

However, a brief search through a collection of standards produces a list of over thirty measures of sight distance (see table below).

Table 1: Some types of sight distance

sight distance 01

 

Notes

  • 10 – Erfassungssichtweite (preception sight distance)
  • 32 – If there is a horizontal sight distance there must be a vertical sight distance
  • 33 – Wahrnehmungsphysiologische Sichtweite (sight distance related to physiological ability ability to perceive)

Comment

One German source discusses the limitations of the physical ability to perceive objects. These have not  been (and cannot be?) modelled in formulae for highway design. Instead, engineers have developed geometric relationships which claim to approximate real-world driver behaviour. For example, one approximation for stopping sight distance (SSD) is based on assumed  values for driver eye height, object height, and braking deceleration.

To complicate matters further, the same brief search through a number of standards indicated that sight distances vary with several other parameters as well, such as:

  1. vehicle type (because of rates of braking, initial speed, driver eye height etc)
  2. whether sight distance is related to (a) successful manoevre or (b) aborted manoevre (ref. 771)
  3. the gradient of the road
  4. the speed difference between overtaking and overtaken vehicles (e.g. ref. 831)
  5. the type of vehicle doing the overtaking (trucks are slower than cars)
  6. the type of vehicle which is being overtaken (trucks are longer than cars)
  7. volume of traffic on the road (the Cambodian interim rural road standards (ref. 1666) gives two sets of values for sight distance depending on traffic volume)
  8. the type of road surface (see e.g. ref. 1035 page 84)
  9. whether desirable or minimum values are being considered (e.g. ref.255)
  10. the type of road (motorway, urban or rural) (e.g. ref. 1617, SSD longer for motorways and refs. 167, 1115)

What the above suggests is that a design engineer should theoretically carry out visibility checks for 180 or more different scenarios, assuming:

  • for three types of road (motorway, urban, rural),
  • at least two vehicle types (cars, trucks) and
  • around 30 types of sight distance

The problem is that most references seem to discuss only one type of road, one type of vehicle and three or four types of sight distance. Perhaps the references over-simplify the question of sight distance, to the detriment of road safety. Also on road safety, it would be interesting to see how many of the geometric relationships of sight distance have been tested to see if they are based on valid theories.

References

112 – Bangladesh:Geometric design standards for RHD; RHD 2000

148 – South Africa, Geometric design guidelines, CSIR 2002

167 – UK, Hobbs- Traffic planning and engineering; Pergamon Press 1974

177 – Germany, Strassenbau, Planung und Entwurf; Verlag für Bauwesen 1997

255 – UK, DMRB 6 section 1 part 1/ TD 9/93 highway link design; Highways Agency 2002

771 – multi-country, ADC / SATCC draft code of practice for the geometric design of trunk roads, 2001

831 – USA, AASHTO – A policy on the geometric design of highways and streets 2011

1035 – Bosnia & Herzegovina, Guidelines for road construction etc, Part 3: Geometrical road elements; Road Directorate 2005

1042 – multi-country, SADC / SATCC Guideline on low volume sealed roads 2003

1088 – SIECA – Manual Centroamericano De Normas Para El Diseño Geométrico De Las Carreteras Regionales; 2004

1155 – Albania, ARDM 2 Road design manual vol. 2 / geometric design 2007

1617 – Wolf et al, Strassenplanung, Werner Verlag 2013

1656 – India, IRC-66-1976 – recommended practice for sight distance on rural highways; IRC 1976

1639 – USA, Bureau of local roads and streets, chapter 28 – sight distance; Illinois DOT, 2006

1666 – Cambodia, Interim rural road standards (final draft); MRD 2006

1783 – USA, Stopping sight distance and decision sight distance, Oregon State University, 1997

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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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