We all know that cars are bad for the environment. The combustion of petroleum-based fuel releases all sorts of toxins into the environment and atmosphere, eating away the ozone layer and poisoning the air. As a result, many devoted environmentalists and supporters of sustainable living have eschewed personal vehicles altogether, opting instead for greener transportation options.

While walking, cycling, and public transit will suffice for most transportation needs, sometimes a personal car is entirely necessary. In these instances, we must decide: ride-sharing or car-sharing? Both transportation strategies are becoming increasingly popular in cities around the world, and both offer solutions to the emissions issue of personal vehicles. However, one is greener than the other, and the truly eco-conscious should know the facts.

Ride Sharing

Uber, Lyft, and other rideshare companies tout an eco-friendly model and confidently position themselves as sustainable and green — but are they? The answer is unclear. Ride-sharing businesses argue that their service is good for the environment for several reasons. Primarily, it allows people without personal vehicles the flexibility of car-based transport, helping more people ditch automobiles. Lower car ownership reduces traffic congestion, reduces vehicle emissions, and reduces the need for nature-destroying roadways and parking lots. The reckoning seems logical and is convincing to many ardent ride sharers.

No ride-sharing business boasts the data to back up these claims, and several researchers are uncertain that this reasoning proves true. The problem is that Uber and other ride-share companies are protective of their data, preventing third-party scientists from analyzing their environmental impact. It’s unlikely that ride-share businesses are hiding anything particularly nefarious; rather, publishing such data might give competitors insight into effective business practices, thereby decreasing individual company’s profits. Because the ride-share industry is incredibly competitive at the moment, it is imprudent to reveal any business secrets. Instead, Uber and others regularly release in-house reports on activity and sustainability, but the methodology of these studies is vague — and the conclusions are inherently biased.

The lack of independent proof about ride-sharing’s eco-friendliness (or lack thereof) should be telling. It is entirely possible that ride share services are inflating their usership numbers with short-lived discounts and promotions, selectively studying higher-efficiency vehicles, or even outright lying about their impact. Because many ride-share drivers work like taxi drivers, it is possible that ride-sharing is only compounding the traffic congestion ad emissions problem, as they drive around cities waiting for fares. Further, for every person who uses ride-sharing responsibly — doesn’t own a personal vehicle, relies primarily on public transit, etc. — there are those who abuse the ride share service, venturing out when they might otherwise have stayed home or ride-sharing alone instead of carpooling with friends.

Until ride share companies release their data, it is nearly impossible to know whether ride-sharing is as environmentally beneficial as ride share companies purport.

Car-Sharing

Meanwhile, many of the benefits of car-sharing are identical to those of ride sharing. It disincentives car ownership by providing access to personal vehicles when they are necessary and encouraging alternative transportation, such as walking, cycling, and public transit, whenever possible. There is one obvious major difference between ride sharing and car-sharing: Drivers.

In ride-sharing, users hail drivers through mobile apps, much as they would a taxi. Thus, drivers often roam city streets, increasing emissions and clogging roadways. Conversely, a car-sharing service maintains a stationary fleet, from which members can rent vehicles for hourly or daily use. While they await use, car shares do not release harmful greenhouse gases — or emissions of any kind — and they do not add to traffic congestion. Further, car-sharing fleets are more flexible than ride shares. Ride-sharing drivers invest in their own vehicles, and few drivers can afford brand-new, top-of-the-line, energy-efficient models. Meanwhile, BMW is working to create an all-electric fleet using its i3.

Even more, early research on the environmental impact of car-sharing has already been released. In Seattle, car sharing programs have reduced cars on the road by more than 4,500; in Berkeley, CAcollege students were 40 percent less likely to purchase a vehicle thanks to available car shares. Plus, car shares typically only spend a few hours parked in one location, reducing the need for massive parking structures and lots and increasing space for green areas, like parks.

While the imminent introduction of driverless vehicles might shake up the debate, for now, car-sharing seems to be the obvious winner in sustainable driving alternatives.