Using transit requires both push- and pull- factors. The latter includes a system that is efficient (i.e. easy to use, quick, and inexpensive) as well as comprehensive (i.e. fulfilling a large share of transportation needs). That is to say, if I can take transit to and from work, but I need to drive anywhere else, the cost of transit commute is larger than the marginal cost (i.e. additional increase) of automobile commute. In that situation it makes little sense to reduce the number of automobiles per household. Otoh, if we can use transit to most household transportation needs, reducing the number of autos per household makes sense.
The cost of auto transportation are the push factor. Gas price is one of them but not only one. Other include traffic jams, maintenance costs etc.
Wojtek