drivers who crashed after falling asleep did not detect any signs of drowsiness before the crash

Half of

Explanation:

It has been established by a survey or research that about half of drivers who have given reports that while driving they fell asleep.

Sleeping while driving can be a major cause of accidents and a great risk to both the driver and other road users. It could be caused by lacking enough sleep, sleep disorders, medications.

this doesn't support enough information to answer the question.