teristics but differ in the built environment (e.g., a compact, mixed-use development versus a traditional, sprawling suburban development) in a quasi treatment control group, if such a pairing can be found, is one way of handling comparability issues. Over time, as the number of reliable studies drawn from many metropolitan areas and settings accumulates, the external validity of research results should improve.

A final issue relates to whether the results of any of the studies would apply in the future. Aging of the population, growth of immigrant populations, and the potential for sustained higher energy prices in the future and new vehicle technologies could result in development and travel patterns that differ from those of today, topics that are elaborated in Chapter 4.

This section reviews in turn five comprehensive reviews of the literature produced over the past two decades; several more recent studies; and studies focused specifically on travel effects of transit-oriented development, compact development and urban truck travel, and estimation of the effects of compact development through modeling.

Over the past two decades, numerous studies have been conducted that have analyzed travel behavior while attempting to control for measures of the built environment and socioeconomic variables that also influence this behavior. Fortunately, noted scholars have conducted five comprehensive reviews of this burgeoning literature (Badoe and Miller 2000; Crane 2000; Ewing and Cervero 2001; Handy 2005; Cao et al. 2008).

Crane (2000) categorizes studies by type of research design and assesses study results in light of the strengths and weaknesses of each approach. Badoe and Miller (2000) summarize the empirical evidence concerning impacts of urban form on travel but also look at mode use