Physicist: In terms of things like space travel, the difference between 100mph and light speed is academic. The speed of light, “C”, is woven into the laws of the universe from top to bottom, mostly in the context of electro-magnetism. Changing the speed of light would have profound effects on chemistry and the fundamental forces. For what follows, the speed of light is now C = 100 mph (161 km/h for our Canadian or otherwise foreign readers). Pretty colors: Even at running speed there would be enough relativistic doppler shift to change the colors around you.