The effect is huge. Energy goes up quadratically, so here (4->9) you have more than doubling. Stopping distance goes up quadratically (well, due to reaction tim

The effect is huge. Energy goes up quadratically, so here (4->9) you have more than doubling. Stopping distance goes up quadratically (well, due to reaction time there is a linear component in there as well, but still). And if there is a sudden obstacle that you can just avoid hitting going 20mph initially, you'd hit it with 22 mph if you're going 30 mph initially (assuming no reaction time and constant deceleration). I submit that that's a massive difference: hitting a child not at all or with 22 mph.