The Longitudinal Study of Overnight Travel (LSOT) panel data were collected monthly from 628 individuals between approximately February 2013 and February 2014. Each month for one year, participants were asked to document overnight trip tours completed since the last survey including the start and end date, overnight stop locations, stop purpose(s), mode(s) between stops, and travel party. Prior work has already developed mode choice models using the geocoded trip ends for this panel. However, in this project we are interested in the relative amount of CO2 emissions associated with flying versus driving for specific trips. The accessibility by air has already been generated including the weighted flying distance and number of stops between nine pairs of origin and destination airports for each trip using the FAA DB1B. Air trips also include the driving access and egress. Shortest free flow travel time driving routes have been calculated from Google API. The objective of this project is to assess the extent to which people have or have not chosen the lower emissions mode and whether the pattern of optimal or non-optimal mode choice varies with personal, location or trip purpose attributes. Results will inform the extent to which policies to affect modal substitution for intercity travel should be pursued.