The last time I took a trip on a bus, it wasn't pretty. It was merely cheap.

But that was a few decades ago, and now I have good reason to try again.

In a 54-page data-laden study, the Union of Concerned Scientists has concluded that traveling by bus - or, as they term it, motorcoach - is the greenest option.

In every scenario they calculated - from a single person going 100 miles to four people going 1,000 miles - the bus was better.

Trains also came out looking fairly green.

This is all good to know, now that vacation season is nigh, because the impact of a vacation on a family's global-warming carbon emissions can be huge.

Using the example of a Chicago family of four that had been minimizing when it came to the daily commute but was planning to fly to Disney World, the researchers found that the trip's emissions equaled nearly two years of commuting pollution.

Naturally, there are all but infinite permutations. But three key factors determine which mode of travel is greenest: what kind of vehicle you'll be using if you drive, how many people are traveling with you, and how far you're going.

Still, green is one thing. But practicality and price are also important. Who wants to spend most of a one-week vacation on a bus?

The researchers suggest an attitude change: The vacation isn't just the destination, it's about getting there, too. Imagine the scenery on the buses that can now get you to the Grand Canyon and Denali National Park in Alaska.

Closer to home, the comparisons get interesting.

A friend recently invited my husband and me to spend the July Fourth weekend in Boston. We'd go on Thursday and come back on Monday.

I checked into options, and here's how they played out:

Driving 350 miles from my home would take about 61/2 hours one way, not counting pit stops. My Prius gets 50 miles per gallon. Factoring in $3 a gallon puts me at $42 for the round trip, and since there are tolls, I'll arbitrarily up that to $80.

Taking a Greyhound from Philadelphia would take 71/2 hours - comparable to driving - and, with 21-day advance purchase, cost $130 round trip for the two of us. Plus, we'd leave the driving, as they say, to someone else.

Amtrak takes six hours and costs - ouch! - $240 for two.

Flying takes as little as 11/4 hours, but I'm going to add getting to the airport two hours ahead of time and call it 31/4 hours. On, the coach fare for two totaled $520.

So if we go, I'll still drive, especially since my times don't factor in getting to the bus, train or plane. But the bus is a tempting second choice.

The bus industry, naturally, is all over the UCS study and other similar ones.

"By supporting motorcoach transportation . . . you might just help save an ice cap or two," the American Bus Association trumpets on its Web site. (A promotional brochure depicts penguins leaving the Capitol in Washington, headed toward a bus.)

Meanwhile, what a perfect time for them to roll out upgrades.

Greyhound Lines Inc. has launched a new fleet - none in Philadelphia yet - that include three inches more legroom, free Wi-Fi access, power outlets and seat belts.

There's a similarly well-equipped "BoltBus" ( that goes from Philadelphia to New York. Hard-to-get fares are as little as $1. This is because tickets are sold only online and there's no terminal. You catch the bus curbside near 30th Street Station.

Checking on Friday for a bus to New York today, it would cost $10 to $12, depending on the time. Not bad.

The UCS study found, by the way, that the least green way to travel is first-class on an airplane. (They apportioned a greater share of the plane's carbon emissions to the bigger seats.)

This is probably needless information since few people can afford it anyway.

Still, if someone were to offer me a free first-class ticket to Paris, I'd take it. Throw in some foie gras while you're at it.

But as that is unlikely to happen, at least the UCS study provides fodder for my next plane trip. I can feel superior instead of envious when I ogle those cushy seats on my way through to coach.