I bought a motorcycle about three years ago to get better gas mileage and it does. In the summer I get about 40mpg on the bike compared to 18 for the truck. That being said, if I could do it over again I would buy a 250 (Ninja 250) instead of a 600 since the 250s get about 65mpg and are still freeway legal. Out of the bigger displacement bikes, the BMWs with boxer engines seem to get the best mpg.
As for motorcycle safety, my recommendation would be to take a Motorcycle Safety Foundation (
Motorcycle Safety Foundation) safety course in the US or a Canada's Safety Council course (
Welcome - Canada Safety Council's Gearing Up - Canada's National Motorcycle Training Program) in Canada and wear the proper gear (helmet, leathers, boots, gloves). There are definitely some scary drivers to avoid. I saw someone texting while driving a couple of weeks ago. Other drivers often misjudge the speed of motorcyclists and they will turn left in front them. Other drivers can't be blamed all the time though. According to msf approximately 50 percent of motorcycle fatalities only involve the motorcyclist.
I am sure that my motorcycle is fairly environmentally dirty. Even though it is the California version it has carbs and does not have a catalytic converter. When I stop, I can almost always smell exhaust and some fuel. Many of the new motorcycles (CA model) are fuel injected, liquid cooled, have cats, and meet both CARB and Euro III requirements.