Previous research suggests that the physical environment is an important influence on active commuting, and the distance of the daily commuting journey has been shown to have a strong and consistent
influence on travel behaviour.79Distance between home and work can be assessed in several ways: by
asking people to report the length of the route, by modelling a likely route or by measuring the length of the actual route. Although distance to work may be relatively accurately recalled because the journey to work is made regularly, self-reported distance may still be subject to recall bias. When modelling routes using a GIS, it is often assumed that the shortest available route is used with the aim of minimising travel
time,80but this may be different from the actual route used, for example if commuters take children to
school on the way to work or go shopping on the way home. The choice of route may depend on several factors, including the social context in which household decisions are made as well as individual preferences and physical constraints. One way of capturing data on the actual routes followed by commuters is to use GPS devices. Although these are increasingly small, unobtrusive and affordable, the participant has to remember to wear the device and recharge it each night, and considerable effort is required to clean, process and analyse the resulting data. To explore the implications of relying on modelled routes instead of collecting GPS data from a large number of participants, we investigated the differences between distances estimated on the basis of GIS-modelled routes and those derived from
actual routes recorded using GPS.81
Methods
We derived distances representing the shortest route available between participants’ home and work
locations with the help of the route networks created in the GIS. For participants who reported having used the car, we used the car route network to model the shortest possible route from their home to their workplace. For participants who reported having walked or cycled, we used the pedestrian and cyclist route network to model the shortest possible route. Distances estimated on the basis of GIS-modelled routes thus reflected the actual modes of transport used by each participant.
Actual routes were obtained from the GPS receivers worn by a subsample of participants in 2010 and 2011. Briefly, this involved extracting GPS data for the relevant trip in the GIS, as well as any intermediate destinations that were visited en route, with the help of background mapping and aerial photography. Scattering of data points resulting from signal error or reflection from buildings was removed manually. We used multilevel mixed-effects generalised linear regression models to assess the concordance of GIS-modelled and actual commuting routes; in the published paper, we also compared other features
of the modelled and actual routes travelled between home and work.81
Results
We found that the actual routes taken by commuters often diverged from modelled routes, sometimes substantially. On average the actual routes were 27% or 4.3 km longer than modelled routes, but this summary result conceals important differences depending on which mode of transport was used. Actual routes were even longer among those who travelled to work by car, either singly (35% or 6.5 km) or in combination with walking (25% or 6.2 km) or cycling (26% or 5.0 km). In contrast, the lengths of actual routes followed on cycling trips were closer to those of the modelled routes, with an average difference of 23% or 0.9 km. Actual walking trips were 13% or 0.2 km shorter than modelled routes, suggesting that participants who walked to work may have used cut-throughs and other paths not present in the route networks used to model routes. Agreement between route lengths was highest for walking and cycling, with concordance coefficients above 0.93, and lowest for trips made entirely by car, with a concordance coefficient of 0.44.
Taking as an example one participant who travelled 20 km from work to home by car, the shortest route modelled using the GIS underestimated the actual distance travelled by 79% and overlapped the actual route for only 15% of its length. For a second participant who walked 2.2 km from work to home, the route was more similar in length to that modelled using the GIS but only 19.2% of the route overlapped the modelled route. Figure 3 provides a graphical example of this type of divergence between actual and modelled routes.
Interpretation
Modelled routes and actual routes can differ substantially. Many commuting journeys are not made with the aim of minimising travel time by choosing the shortest distance, but instead often incorporate other destinations. The next chapter offers additional insights into the motivations for route choice, either to avoid undesirable features of the shortest available route or to experience desirable features of
the alternative route. Nevertheless, the shortest modelled route to a destination– whether this is a
workplace, or the nearest bus stop or access point to the path on the busway– may be a reasonably
accurate estimate of the actual distance followed, particularly for walking or cycling trips, and can serve as an indicator of the accessibility of that destination for a given participant (see Chapter 5, Linking environmental change with travel behaviour change in commuters).
FIGURE 3 Comparison of actual and modelled commuting routes. The actual route taken– by bicycle in this example– is longer than the modelled route and overlaps only in parts. In practice the hypothetical participant takes a less direct route, travels along quieter roads and passes fewer destinations than the modelled route would suggest.