I think that the cell phones are the primary reason, but a secondary reason is that motor vehicles have become very "comfortable" to drive with all the new technology. In new cars, you can barely feel the road, and you can barely hear anything outside of your car. The result is that it's almost like being in a "cocoon" without any outside influences. People really do not know how fast (or slow) they are driving anymore, because it "feels" the same at 70 mph or 35 mph.