The thing is that there's not a 1:1 relationship between a "risky" maneuver and causing an accident. An example could be an F1 driver who takes curves at 20mph over the recommended speed in a sports car. Compare this mentally to a Prius driver who drives under the speed limit in the left lane. The Prius driver is "being safe" but causing a long line of tailgaters and people passing unsafely on the right, as well as agitating people and causing them to drive more aggressively. I doubt that the first few iterations of this system will take psychological effects on other drivers into account (if they ever consider that / have a profit motive to do so at all).

Indeed, a proportion tells us something about the probability of making a driving mistake. A boolean tells us little in comparison.