The Department of Transportation will soon be using Zipcars instead of city-owned vehicles, Mayor Bloomberg, Deputy Mayor Stephen Goldsmith and Transportation Commissioner Janette Sadik-Khan announced at a press conference yesterday. The initiative is intended to reduce unnecessary driving by DOT employees and could yield significant savings if expanded to the city’s entire passenger vehicle fleet. Symbolically, the city is also sending a message that owning a car might not be a wise financial decision.

Three hundred DOT employees will share 25 Zipcars during the workweek, which they’ll reserve online. Those 25 vehicles will replace 50 that had been owned by the department. At night and on the weekends, regular Zipcar members will have access to the vehicles, which will be stored in private garages in Lower Manhattan.

The car-sharing idea has been in the works for a while now. DOT put it out for competitive bidding a year ago and the plan made it into Goldsmith’s cost-cutting program in July. Now it’s ready to go.

At yesterday’s announcement, a lot of attention went to the savings and efficiency gains that car-sharing could bring. The city’s fleet is currently “a patchwork of standard operating procedures and tracking mechanisms, some of which are still paper-based,” said Bloomberg. This DOT pilot, he said, would save the City $500,000 over four years, and could be scaled up to the entire passenger vehicle portion of the city’s 26,000 vehicles.

Cities like Washington, D.C. and Philadelphia have reaped big savings by contracting with car-sharing companies for their fleets.

Bloomberg also promised that switching to car-sharing will “reduce the congestion on our streets and the pollution in our air.” That may be the case with consumer car-sharing, which leads people to weigh the price of each car trip against other options, but it’s not quite clear whether switching to Zipcar will help DOT reduce the amount employees drive, since the agency is paying a fixed amount already.

Bloomberg said the main effect would be to discourage employees from commuting with agency vehicles — an improper use of city cars. Restrictions on when city workers can use the shared cars will keep most of them off the streets during peak commute hours. The mayor also hypothesized that having to walk to a public garage might be a disincentive to take unnecessary trips.

We’ll know the effect soon enough. Sadik-Khan said that GPS would be installed in the city’s Zipcars and used to measure whether the program is actually reducing driving. Bloomberg suggested that DOT also use the GPS data to find driving trips that could be done on transit. “If it turns out the subway is better, or the new M15 bus,” he argued, “maybe that’s the way to go.”