Have you ever found yourself behind the wheel, trying to express gratitude to another driver, but you didn't know how? Say you're in your car, windows up, driving down the road, speed untold, and someone lets you change lanes, or gives you the go-ahead at a stop sign. You want to scream "Thank you!" at the top of your lungs, but you realize that probably won't do the trick. Thankfully, William Safire of The New York Times has compiled some alternatives. Here are ten signs full of gratitude.