Below I posit some directions for research in transport and land use. Comments welcome.
1. We need more panels and time series and fewer cross-sectional analyses. If we want to establish causation, we need to look across time, otherwise, we are stuck simply with correlations. [And as we know, correlation is not causation]. We need data that examines the evolution and dynamics of transport and land use systems. I have not quite come to the conclusion that all analyses must be temporal (that is rejecting any atemporal analysis), but I am really tempted to do so as a reviewer.
2. We need to improve the scientific rigor of our research. The discipline is ripe for continuing meta-analysis to establish the magnitude of effects, and to reduce the range of estimates (and explain the range that exists through different underlying causal factors).
3. We need to more systematically consider network structure when looking at explanations of travel behavior. This includes measures of topology, morphology, and hierarchy. The measures that have historically been used have been relatively easy to estimate, but don't get at the gestalt of the network as an integrated system.
4. We need to systematically look at the difference between travelers perceptions of how systems operate and how long are travel times, and what we analysts measure. The differences can be systematically explained, at least in part, and people of course make decisions based on how they think the world works, not on how we think it does. We could then examine why perceptions differ from measurements, how much is simply differences in linguistic interpretation (when a trip begins and ends is somewhat ambiguous, e.g.), and how much is differences in time perception, and how much is "rounding" error, and how much is strategic to either impress with the length of the commute (which brings to mind the Four Yorkshiremen sketch) or to exaggerate in order to get sympathy or a policy response.
5. We need to increase the inter-disciplinarity in the study of transport and land use research, with planners, geographers, engineers, economists, and others working together looking at these problems.
6. We need more international and historical cases in the field to build towards a general truth. Reasoning is both inductive and deductive, but so much of what we are doing is complex, one often cannot simply derive from theory whether a change will lead to more or less travel, it depends on parameters, for instance. the fixed costs of engaging in a trip vs. the variable (and non-linear) costs of travel.