I have a simple rule for you when you're on foot - don't every try to pass comment on how fast you think a vehicle is travelling, because you're wrong. Trust me on this - unless you have a radar or laser speed gun, you have no idea how fast any vehicle is going. Not a car, train, bus, not even a scooter with Clarkson riding it.
This can be illustrated by what happens to most drivers from time to time. For example, I'll be minding my own business, driving through a residential neighbourhood - mostly on my way to or from my house - and someone will come running out into the road, flapping their arms and screaming about what an evil person I am for speeding in their neighbourhood. At that point, as a driver, I've pretty much already lost the argument, because no matter how slow or fast I was actually going, no matter what my speedometer said, the pedestrian who just tried to get themselves run over will be utterly convinced that I was travelling at a hundred miles per hour.
This most commonly happens in areas where the defined speed limit is ultra low, like 20 to 25mph. It doesn't matter if you are doing 15mph, as far as speed-judging pedestrians and homeowners are concerned, you're blitzing through their neighbourhood at suicidal / breakneck / irresponsible speed (take your pick).
You can try logic, math, physics, basic conversational explanation and any other technique you choose, but as I said - because you're a driver, you're in the wrong and you'll never win this particular argument.
I've found over the years that the best course of action is to not run them over when they flap into the road in front of me, keep my windows up and just drive around them at whatever speed I'm going. While I'll be the first to admit I break speed limits on the motorways, residential areas and school zones are my speeding taboo.
My advice to you if you're on foot and you think you see a car speeding is to not run into the road to try to confront the driver. To most of us that would be common sense, but it seems a lot of people still can't figure that out on their own.