Since that's what I said, let me expand on it so maybe you'll possibly get the point. That quote applies more to a driver. If you are cruising at 15 mph and the car is doing 40, then your combined speed is 55. That's reducing the amount of time for the driver (operating a much larger body of mass than you) to check their blindspot to avoid you. Or if they don't bother checking and just swerve, it potentially increases the SNAFU factor exponentially. What if they collide with a bus load of kids just to avoid you?

If you are doing 15 with traffic doing 40 in the same direction, then the driver has additional time to react, as the new combined speed is 25 mph. This would allow a driver to come up behind you, slow down to check for a break, then safely accelerate past you.

Grab a buddy and find a stretch of road where you can drive at 50 mph without disturbing anyone (industrial parks on the weekends). While one drives, the other takes a stopwatch. At a predetermined point, hit the brakes- HARD. Compare times at 55 and 25...