There is one additional "safer" factor you didn't calculate. Under the riding conditions you specify if someone does hit you the speed differential comes into play.
Namely if some knuckle head high speed heavy vehicle operator either runs right into the rear of you on your bike or passes so closely there is a sliding physical contact between the right side of their car and your left side and the protrusions such as the mirror, door handle, and trim make multiple impacts with your bike and body along with road rash like burns from the sliding contact with the main smooth body of the car. That kind of "accident" (in quotes for a reason, criminal negligence on the part of the passing vehicle would be more like it) is known as "getting sliced" up here where I live and is a common danger to avoid on the kind of roads you are talking about.
Anyway, long story short if we assume the car is going 60-mph.
----- If a car either rear-ends you or "slices" you on a too close pass and you are going 10-mph the speed of the impact is 50-mph
----- If a car either rear-ends you or "slices" you on a too close pass and you are going 20-mph the speed of the impact is 40-mph
----- If a car either rear-ends you or "slices" you on a too close pass and you are going 30-mph the speed of the impact is 30-mph
Providence-Forbid, if one of the drivers of those faster moving heavy vehicles on the road either plows into you from behind or "slices" you on a too close pass the faster you are going the lower the speed of that impact will be when they hit you and thus the better your chances of avoiding death and/or reducing injuries from the initial impact.
So, provided you are riding on the correct side of the road and not being a salmon (speed gets added rather then subtracted in that case, which is part of the reason you shouldn't ride like that) then you have an additional safety cushion if you are moving at higher speed if someone actually does hit you.
If you ride correctly with traffic the faster you ride and close the gap between the speed of other traffic on the road the better off you are if they hit you. The reverse is true as far as you hitting them (be careful when drafting cars, sometimes they can brake before you can and if you don't leave enough room you can rear-end them, the only at-fault collision I have been in on a bike was when I rear-ended a vehicle in front of me and split a foam bike helmet in half when my head hit the rear of their vehicle, and I was riding a pedal only bike !!!).
Last edited by turbo1889; 07-14-13 at 12:50 AM.