These maps were derived by calculating the percentage of workers 16 and over who reported that they taken public transit to work in 2000 and dividing this number by the percentage of workers 16 and over who reported they had taken public transit to work in 1990. The result (numbers running from 0 to 16) was expressed as a percentage of change (.5 = loss of 50%; 1 = no change; etc.). Because percentages were involved in the first calculation, the change from one decade to the next should not have been affected by a change in population, but it should be remembered that  transit data come from the Census Bureau's sample survey (part 3A, the long form) and are not as reliable as the 100% data; and  in parts of the suburbs only a very small percentage of workers took public transit to work. As a result, the suburban data tend to be volatile. An increase from 1% to 2% would count on these maps as a 100% gain; a perhaps more significant increase in an urban tract from 40% to 50% would be treated as a gain of 25%.
Tracts with fewer than 50 people or those for which no mode-of-transit-to-work figures were reported for either 1990 or 2000 are omitted from this analysis.