The transportation sector is responsible for a third of greenhouse gas emissions in the United States, 95 percent of which is made up of climate-altering carbon dioxide. Most of the major metropolitan areas in the country are plagued by traffic congestion and poor air quality. Building more roads to accommodate the insatiable craving for driving space, while ignoring the negative side effects, is like serving sweet cake to a sugar addict. Here are your five best bets for greening your carbon tireprint.

1) Leave your car at home. Carpool to work instead of helping create traffic jams. A great way to lessen your carbon footprint is to reduce the number of cars on the road with three empty seats. Make new friends, save money and possibly qualify for employee benefits.

Carpooling, particularly in areas underserved by public transit, is a way to get people to leave their cars at home. Commuters headed in the same direction can travel together and save money by splitting the cost of gas, tolls and parking. Some insurance companies, such as Geico, offer discount plans for rideshares. Every rideshare helps remove four cars from the road — the equivalent of planting 4,000 trees. A once-a-week shift to carpool can reduce a commuter's carbon footprint by 20 percent.

Carpoolers can alternate driving each week to lessen the burden on any one person. Establishing clear boundaries about what's permissible while sharing the ride will help avoid later conflicts. Set rules to clarify smoking, eating or cell phone chatting preferences on the ride. Advance discussions of how you'll split trip costs can also prevent future misunderstandings. Meet your neighbors and build local camaraderie.

Some employers offer incentives, such as priority parking spaces, to encourage ridesharing. In many cases, carpoolers can also cut travel times by using HOV, or high-occupancy vehicle, lanes. There are also other discounts that make ridesharing attractive, such as reduced toll payments and gas expenses. Carpooling cuts travel times for everyone on the road by easing congestion, but it specifically lets those using HOV lanes bypass major traffic jams.

Numerous websites provide ways to connect to other commuters searching for or offering a ride. First, find out if any co-workers live in your area. A carpool may already exist that you can join. Companies sometimes have bulletin boards where workers can post messages to find each other and arrange rides. A 60-mile daily commute in a two-person carpool saves $4,387.50 annually versus commuting alone.

Websites like Craigslist are helpful for finding rideshares. Rideshare, Carpoolconnect, NuRide and Zimride are examples of sites that can help you find a ride or start one yourself. Zimride has an application on Facebook to make connecting to potential carpoolers easier. NuRide has a video tutorial that offers step-by-step instructions on how to set up a carpool.

2) Use public transit. Get a monthly pass to make your commute more cost-effective. Check Google Maps to arrange your transit trip. Don't be scared to transfer between modes. It might be faster to reach work by, say, bus than sitting in a traffic jam.

Buses, ferries, trains, trams and trolleys are the forgotten gems of mobility in many parts of the United States. An individual who switches to mass transit in a single day can reduce carbon dioxide emissions by more than 20 pounds, shedding more than 4,800 pounds of CO2 in a year (PDF). Compared with private vehicle use, for every passenger mile traveled, public transportation produces 95 percent less carbon monoxide, 92 percent fewer volatile organic compounds and nearly half as much carbon dioxide and nitrogen oxide.

Public transit can be comfortable enough to lure commuters out of their cars. Certain areas, like Los Angeles, have special dedicated lanes that allow buses to reach high speeds between stations, mimicking underground subways in efficiency. Driving, after all, can lead to increased levels of stress and road rage, especially compared with commuting by rail (PDF).

Employers may offer cash-out opportunities that reward transit users with monetary enticements for giving up driving to work and forfeiting a parking spot. Other benefits may include TransitCheks, which provide tax-free savings with every transit commute — made possible by the U.S. Department of Transportation's Commuter Choice Program. Besides TransitCheks, employers may offer other types of travel allowances. Contact your human resources department to find out what incentives exist that can motivate you to embrace transit over driving.

Google Maps now offers a function that allows users to map transit trips by typing in the origin and destination in the search bar. The results even include alternate route suggestions, service timetables and trip duration based on different mode-mixing schemes. Hopstop is another program that conveniently produces directions based on bus, subway or walking preferences. Other regions have their own custom-made trip planner, like King County in Washington state, which includes the Seattle metro area. It has functionalities like determining the route with the fewest transfers and handicap accessibility.

Photos: Texas T/Flickr and nathangibbs/Flickr

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How to green your commute: Carpooling and public transit
Traffic driving you crazy? Let MNN get you on track to a more astute commute with these carpooling and public transit tips.