Our roads and highways are already crowded. But just imagine the traffic when our state population doubles in 50 years.  Not only will our commutes increase, but cars idling in stop-and-go traffic pollute up to three times as much as free-flowing traffic.

So how can we protect the earth and get to work on time?  While train systems, denser neighborhoods, and biking are all great options, we look at four vehicle-based alternatives below.

High Occupancy Vehicle lanes: By providing less congested lanes, HOV regulations encourage carpooling. San Francisco's Spare the Air program estimates that adding one more passenger to all commuting cars could save 600,000 gallons of gasoline and cut 12 million pounds of carbon dioxide emissions per week. Between 18% and 23% of drivers in Puget Sound HOV lanes said that the lanes were their incentive to share the drive.

High Occupancy Toll lanes: WSDOT is considering this system, which would allow a limited number of single-passenger cars to pay to use HOV lanes during midday. Carpools, vanpools and buses would still drive free in HOT lanes. Allowing solo vehicles in the HOV lanes may provide convenience for a few, but no real environmental benefit.

Bus: The nation's average transit bus carries only 9 passengers (counting both peak and off-peak travel). What's more, buses are stop-and-go traffic by nature, emitting extra pollution and worsening congestion on roads. But every switch from car to bus helps boost bus efficiency, and proposed bus rapid transit could attract new riders. While an empty bus is less energy efficient than an SUV, a bus with 27 passengers can be twice as efficient as a Prius.

New Roads: Congestion is a tricky problem and not easily solved by building new roads. Although the new tarmac may temporary alleviate traffic, in the end it simply encourages more driving and CO2 pollution. In fact, a recent Sightline Institute study found that every new mile of highway results in a net increase of more than 100,000 tons of carbon emissions over 50 years.

Our conclusion: No matter your vehicle of choice, increasing the number of passengers per vehicle and reducing solo car trips is the true way to ease congestion, reduce emissions and increase efficiency. Try to replace one solo trip this week with a bus ride or carpool.