The data used to estimate the models of equations (8) and (9) in Chapter 4 come from the 1990 Census Transportation and Planning Package (CTPP) Urban Element for Chicago. The CTPP, which is derived from the 1990 Census long form survey of the population, provides three sets of data (referred to as CTPP-1, CTPP-2 and CTPP-3) on residents and workers aggregated at small geographic levels. A data dictionary for the CTPP is found in the Appendix.