What pose a particular danger to motorcyclists and can compel them to change their position?

Gravel patches  are a particular danger to motorcyclists and that can compel them to change their position.

Explanation:

The gravel under a high-speed motorcycle's tires will trigger it to skid and slide. Motorcyclists therefore always enjoy riding clean and clear roads. If there is gravel scattered on the roads, motorcyclists are advised to lower their speed and pass the gravel patch at the minimum speed feasible. This is because a motorcycle's tires are not as wide as a car's and it must be balanced on only two wheels, which isn't the case with a car.

coil wire, increase current, put iron inside the coil

.) the observation by semmelweis that more mothers died of childbed fever when their doctors also worked in the morgue