The use of GIS to model routes may be acceptable when an approximate estimate of travel distance is required or when estimates of the
features of potential routes that could be taken are needed. This is particularly relevant for active commuting, where actual routestravelled may show high levels of agreement with modelled shortest routes. However, the predictability of the commute route clearly
depends on the mode of travel used. Therefore, public health researchers should not rely on uncritical assumptions regarding the validity
of GIS-modelled routes as a proxy for the actual routes followed by commuters, particularly for those travelling by car. If we are to quantify
exposure to environmental features along routes followed to and from work, or accurately estimate distance travelled by car, our results
emphasise the need to obtain information on actual commuting behaviour. To obtain accurate estimates of environmental contexts in
which behaviour actually occurs, the use of GPS data is recommended, as is the consideration of appropriate characteristics of the
local area.