As national gas prices top $4 (almost $4.20 in NYC and CT), the car-sharing service Zipcar is launching a new promotion this summer to encourage individuals to give up their personal cars and walk, bicycle, ride transit, or use Zipcar instead.

Zipcar’s Low-Car Diet kicks off July 21st in all twelve of its North American cities. In exchange for relinquishing their car keys, participants will get transit passes, Zipcar membership, and drive time credit.

Zipcar is looking for 30 to 50 people to sign up for the program nationwide, and will select participants based on their commitment to driving less.

At the conclusion of the 30-day program, LCD participants will “weigh-in,” reporting on how they changed their behavior and calculating their gas and emissions savings over the period.

For more information on the Low-Car Diet, email lcd@zipcar.com.