## Values for driver eye height

Driver eye height is one of the parameters used in checking sight distance (stopping sight distance for example). Some references only quote a single value, the eye height of drivers of cars. The argument for this is that truck drivers usually sit higher than car drivers and so they can see farther.

Other references argue that in some circumstances sitting higher can be a disadvantage – for where there is an overhead obstacle on sag vertical curves (e.g. a bridge over the road). Here, a truck driver with a higher eye height will more likely have his line of sight line blocked by the bridge. Several standards therefore quote an upper (truck driver) eye height and a lower (car driver) eye height. Design engineers have to test the available sight distance over this range (or envelope) of values. This also gives us the idea of a range of values and a minimum value.

An envelope is a two-dimensional object, but roads are three-dimensional. There may be several traffic lanes, and sight distance should be checked for each of them, over a range of values which create a “box” shape. And of course there are more than just two types of vehicle on most roads, and some parts of the road may be restricted to vehicle types other than cars or trucks. Obvious examples are bus lanes, cycle tracks and pedestrian footpaths. Geometric design guides should give figures for the driver eye height of other vehicle types.

Driver eye height – consistency over time

The AASHTO green book 1994 edition suggests an eye height for car drivers of 1,08m and refers to studies from 1978. The 2011 edition suggests 1.07m and refers to a study from 1997. A 1974 UK source (ref.167) refers to a figure of 1.05m. So there has been basically no change over the 37 years from 1974 to 2011.

Different values quoted for one country

It is possible to find different values for eye height quoted in different sources from the same country. For example in Peru, three sources published between 2001 and 2008 give 4 values of car driver eye height (ranging from 1.07m to 1.15m) whilst the Albanian standard (ref. 1155) quotes two values (1.10m and 1.0m).

Different values quoted for different countries

You might expect to find different values for driver eye height since the people in some countries may be taller than the people in other countries. However  I doubt that the writers of reference manuals go into such detailed research. What does seem more likely is that the design guidelines from different countries generally give more or less the same figure for car driver eye height (see e.g. table 1 – figures are in metres).

Suggested Values

The notes above argue that geometric design standards should give values for the “driver” eye height for different types of vehicle, and that the values should be a minimum and a range. As a starter I suggest the following values (source details available on request).

References

112  – Bangladesh: Geometric design standards for RHD; Draft version 4, October 2000

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

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

1155 – Albania: Albanian road design and construction standards, ARDM 2: Geometric Design; 2007

1597 – Afghanistan: Afghanistan Rural Roads Manual version 2, Ministry of rural rehabilitation and development; 2013

1635 – Australia: M.G. Lay, Handbook of road technology (2nd edition) vol. 2; Gordon and Breach, 1990