Every fall, millions of college students flood their campuses as they return from summer break, hauling with them clothes, computers, books, booze, furniture, televisions, microwaves -- and, in most cases, their cars.

But as the popularity of car-sharing programs, like Zipcar, grows on campuses across the country, more students are leaving their own cars at home and joining a service that The New York Times Magazine said is "bent on altering the primal bond between Americans and their vehicles."

The leading car-sharing service in the world, Zipcar, currently has cars available at 120 different schools in North America, in addition to its individual- and business-based car sharing options on its Web site.

The way Zipcar works on campus, as University of Michigan Alternative Transportation Coordinator Brian Pawlowski explained in a recent interview, is U-M students, faculty or staff first sign up for Zipcar using the University's personalized Web page on the Zipcar site. After receiving your "Zipcard" in the mail, which acts like a credit card for the service, you go back to University's Zipcar Web page, go to "Find Cars," pick out a car at one of the five parking locations in Ann Arbor and reserve that Zipcar for the day and duration of time you'd like.

A window card reader in a Zipcar on the U-M's Central Campus. Zipcar members wave their membership card by the reader to unlock a Zipcar. Photo by Andy Kroll

When the time comes to use the Zipcar, Pawlowski continued, you go to the car's parked location (all of which are next to public bus lines in Ann Arbor, eliminating the need to drive to the Zipcar). There, you wave your Zipcard in front of a scanner on the car's windshield, the doors unlock, the keys are inside and off you go.

Here on U-M's campus, Pawlowski, who oversees the campus Zipcar program, said it has only grown since starting in Ann Arbor just over two years ago. Membership in the program has increased to a thousand members affiliated with U-M (that doesn't include many other Ann Arbor residents not affiliated with U-M but who are Zipcar members), and the number of cars in the University's Zipcar fleet has increased from six to 13, he said.

And with the membership age recently lowered to 18, Pawlowski believes even more students will join the car sharing program.

"I think it's only gonna grow," Pawlowski said. "Each year we reach more students that are coming to campus for the first time. We're reaching students with orientation and fliers in the mail."

The driving restrictions on Zipcars might dissuade some on college campuses from joining, however.

As Pawlowski said, the mileage limit for a Zipcar is 180 miles per day. Which means that a car is ideal for day trips to Lansing (65 miles one-way), to Detroit (43 miles) or to Toledo, Ohio (60 miles). Trips exceeding that 180 miles per day limit cost either $0.45 or $0.55 for each extra mile.

The cost presents another obstacle for college students, too.

For a University of Michigan student (and many other college students depending on the school), there's a $35 annual fee to be a Zipcar member. When renting a vehicle, the cost is $8 an hour or $60 a day. Those rental rates include gas costs, as a separate Zipcar credit card (not the membership card) comes with each card and is used to pay for all gas.

Maintenance is included as well, and the types of cars available range from Ford Escapes and Mini Coopers to BMWs and Mazda 3s; in some locations, SUVs and minivans are available, too.

A Mazda 3 parked at one of the five Zipcar locations in or near the University of Michigan's campus. Photo by Andy Kroll

Granted, a typical parking pass in a U-M lot here in Ann Arbor costs upwards of $1,000 per school year -- on top of all the other costs of owning a car -- so it ultimately comes down to how much a student intends to drive throughout the year.

Thomas Chan, 24, a former University of Michigan graduate student who has used Zipcar on U-M's campus for about a year and doesn't personally own a car, said he uses the service about once a week, to go grocery shopping or for other appointments.

For someone used to spending no money on cars at all, the service can seem "a little bit pricey" at times, Chan admitted. But when compared with paying for gas, car insurance and maintenance costs, he said he thinks Zipcar is the better choice.

And though the process of reserving and using a Zipcar is easy, he said the only inconvenience he's encountered with the service was waiting a day or two for a car to become available.

Of course, there will always be tension over a service that encourages people to share cars and not buy them, especially in the auto-centric southeast Michigan region, home to Detroit and the Big Three automakers. Should car-sharing services continue to catch on throughout the country and the world, it would pose yet another problem for struggling car manufacturers already recording major losses due to dismal sales.

But, on the other hand, the environmental impact of Zipcar is praiseworthy.

By encouraging people to share a small fleet of cars in each city or on each campus instead of individuals driving their own cars, the company has reduced the number of cars on the road. That means decreased carbon emissions and gas consumption -- both critical changes needed to reduce dangerous greenhouse gas emissions. (Zipcar estimates that each of its cars takes roughly 15 to 20 other cars off the roads, though a company press fact sheet offers no methodology for that figure.)

So will Zipcar's car-sharing model change the way we travel? Even the way we live? The company estimates that it has 250,000 members worldwide -- not an impressively large number, but it continues to grow each year as more people join the car-sharing movement. Thus, whether Zipcar's car-sharing model becomes the norm for transportation remains to be seen.

What is for certain, though, is car sharing, despite its cost and potential inconveniences, is a significant, crucial step -- leap, even -- on the way to a more green, environmentally sustainable way of living.