Origamist said:
In the example cited by blockend we're talking about descending at up to 35mph or so - at these speeds I'm reluctant to take my hands off the bars when I know there's a potential threat up ahead, at lower speeds it's less of problem and I occasionally use hand gestures to warn drivers. However, as I've had these misinterpreted/misunderstood in the past, you need to be careful.
Sitting up, freewheeling, arm movements, looking behind are all good ways of getting attention, but in some situations you'll go past the point of no return and you are at the mercy of a car pulling out.
So there are multiple hazards waiting. The guy knows them.
At the bottom of a hill, there is a junction where motorists are known to pull out.
For one, I wouldn't allow myself to get to a speed down the hill where stopping is not possible with only one brake ( the rear ).
Second, the OP has implied he makes his decisions based upon 'fast roadies tend to go flat out'. A 'hope for the best' situation. NOT good.
On the Walsall Road between The Scott Arms and Walsall in N Brum, there is a downhill where cyclists could get the 40 mph limit.
At the base of this hill, there was a side road ( now a set of lights ).
Unwary cyclists were often caught out by inattentative motorists emerging from the sideroad.
During the evening rush hour, I would be in the second lane with the cars freewheeling at 40 .
Now the lamps are in operation, I know I will have difficulty stopping if they change to red, so I forsake the thrill-ride and roll down the hill in the nearside expecting to have to stop.
Mostly not but they could....