I'm pretty sure a bike in good condition is capable of stopping much faster than any motor vehicle, given the same speed and road. I could be mistaken, though.
When I've been going downhill with 35-40mph on a racing bike(rarely, I'm not particularly big cyclist), I've been unable to brake nearly as fast as I could on the same road with a car. Theoretically I could, but in practice the tires are very narrow and if braking anywhere near full power on non-perfect (dry, straight) conditions I could lose control while skidding, resulting in a very hazardous or deadly tumble.
I mainly ride mountain bikes (on roads), so that might be my disconnect. The tire-width thing makes sense. I also hadn't realized before this thread that road bikes don't usually use disc brakes. Seems odd.
Disc brakes aren't always used on road bikes, because their advantages aren't excercised much - you generally don't brake so much that heat is a problem; and wheel bending isn't a problem since unlike mountain bikes, you aren't expected to hit stuff that might bend a wheel and continue riding.
For wet conditions, disc brakes would have an advantage on road bikes for others, I'm not sure.
Possibly, but bikes are not generally equipped with ABS and traction control. In emergency scenarios, I would wager that cars have better collision-avoidance capabilities.
