Sight distance is one of the key areas in highway design. This is because the presence (or absence) of suitable sight distance has a large effect on road safety. It would be interesting to look up values for sight distance from different countries and see how they compare. These notes look at “intermediate sight distance” (ISD). One definition for this is:
“A distance equivalent to twice the stopping sight distance, a distance where overtaking could be attempted with reasonable safety” (ref. 80),
ISD is usually twice the stopping sight distance.
Different terms used
One problem is that there is no consistency in terms used – at least, not in English-language documents. There is of course a separate problem when the source documents are in different languages. I tried to create a list of different types of sight distances (see here) – the list is quite long and is still being extended. For ISD it looks likely that the terms;
- Intermediate sight distance
- Continuation sight distance
- Meeting sight distance
- Stopping sight distance – single lane roads
…. probably all refer to ISD.
Roads where ISD is relevant
ISD has a limited application. For example (Ref. 843) says that “ISD is only applicable to undivided 2-way road (single lane each way)” Other sources say that ISD is also used for rural single lane roads.
ISD and different vehicle types
The new Austroads guidelines (ref. 1887) say “The design of all new roads should cater for the sight distance requirements of trucks”, and “Research …. suggest that the sight distance advantages provided by the higher driver eye level in trucks do not compensate for the inferior braking of trucks“. So design standards should specify ISD values separately for different vehicle types, such as trucks, cars, buses, bicycles; but in my research so far it seems that they don’t.
Another interesting point about trucks and sight distance is that Austroads says that ” for design purposes, the braking of an unladen vehicle in wet weather conditions without locking the wheels is assumed“. A bit odd, as a loaded truck would likely have a longer braking distance.
ISD and design speed
ISD is related to design speed. However the design speed can vary with different vehicle types, since for example some countries place lower speed limits on goods vehicles than on cars. The various tables for ISD refer to a design speed for cars, not for other vehicle types.
ISD and type of road surface
Friction values vary with road surface (for example they are different for asphalted and gravel roads), so that stopping sight distance and ISD vary with road surface.
ISD and desirable, minimum values
Austroads applies the idea of “extended design domain”, which has the idea of a range of values for a particular geometric design parameter. With regard to ISD, Finland (Ref. 1159) provides three values of ISD for each design speed
- Ohjearvo tai hyvä (desirable or good)
- Vähimmäisarvo tai tyydyttävä (minimum or satisfactory)
- Välttävä (taajama) (poor (urban))
ISD and headlight sight distances
At night, visibility is often limited by how far you can see with your headlights. This headlight sight distance is a maximum of 150 m (Ref. 80, 981), which is about equal to ISD at a speed of only 60 km/hr (see table below).
The first part of the following table gives some of the values from the Finland source. The second part lists some values for ISD taken from standards for other countries. They are listed in date order. It looks as if the figures for the other countries are quite similar to each other, and also similar to the middle row of the Finland figures (“minimum/satisfactory”).
Table 1 covers a period of 37 years (1976 to 2013).
It looks like “what you see is NOT what you get”. Standards which give data for ISD give figures which have only a limited and not a general application. Perhaps they should be followed by a “health warning” about their applicability, on the lines of:
“The figures quoted for ISD are only suitable for cars travelling on surfaced roads in daylight. They do not apply to other vehicle types, other road surfaces, or cars travelling at night. Roads designed with these values for ISD may not be safe for these other circumstances.”
80 – New Zealand, “shgdm-part-2 Basic design criteria, update April 2003: Transit New Zealand, 2003
508 – Nepal, “Nepal Rural Roads Standards 2012”, DoLIDAR: 2012
843 – Singapore, “Civil design criteria for road and rail transit systems”, Land Transport Authority; 2010
981 – Australia, “Road design guide”, NSW; 1991
1159 – Finland, “Tien suuntauksen suunnittelu” (Standard 30/2013: Road alignment design; Finland Transport Agency; 2013
1656 – India, “IRC-66-1976 – recommended practice for sight distance on rural highways”, IRC; 1976
1822 – Sri Lanka, “Geometric Design Standards of Roads”, RDA; 1998
1887 – Multi-country, “AGRD part 3: Geometric design”; Austroads 2010