1. All metropolitan areas are included. Classifications are based upon 1990 population. A major reclassification of metropolitan areas occurred in the early 1990s, making pre 1990 data non-comparable. As a result, the 1996 population estimate is based upon the previous 1990 classification figure adjusted to account for the percentage increase from 1990 to 1996 under the new classification.
2. Some public transport agencies did not report in 1981, inasmuch as this was the third year of the National Transit Database, which was not yet fully operational. The 1981 ridership data is estimated based upon the change between 1981 and 1997 among reporting agencies, with the percentage of change applied to the 1997 actual data to obtain the 1981 ridership estimate. This adjustment resulted in an estimate 3.5 percent higher than the reported 1981 ridership. The following modes were included: motor bus, trolley bus, tram, metro and regional rail.
3. The National Transit Database reports unlinked passenger trips (boardings), rather than passenger journeys. To obtain passenger journeys, the Nationwide Personal Transportation Survey 1995 transfer factor of 0.8 was applied.
4. To obtain market share, it was necessary to develop an estimate of passenger kilometers traveled in private vehicles. For 1997, this was achieved by applying the average daily vehicle kilometers per capita for each population category to the total population of the category. The 1981 estimate was obtained by applying the more conservative of two available estimated of the increase in the average per capita vehicle kilometers. Each figure was multiplied by 1.61, which is the average urban vehicle occupancy. The daily passenger kilometer estimate for each population category was then multiplied by 300 to obtain an annual estimate (this is a conservative annual multiplier). The public transport market share is the percentage of annual public transport passenger kilometers divided by the total public transport and private vehicle passenger kilometers.