What you want is the distance but that's not the risk you're being exposed to. As soon as you enter a plane you are being exposed to risk, like a radioactive element, regardless if the plane travels fast, slow or even in circles. Only when you get off the plane does your exposure to this risk change. The distance you traveled in the meantime had no direct effect. The fact that cars or buses take much longer to arrive means you have to factor in the additional hours of exposure to their risks, so it still makes a material difference how far you need to go, but the more direct relationship is the amount of exposure to the radioactivity.