Final answer:
To ensure safe driving, one must have at least 8 consecutive hours of off-duty time before driving after being on duty for the maximum allowed hours. Sleep deprivation can greatly impair driving performance, as shown in studies where drivers are less capable after being deprived of sleep.
Step-by-step explanation:
You must have at least 8 consecutive hours of off-duty time before driving after you have been on duty for the maximum allowable hours according to transportation regulations. The correct option to complete the sentence would therefore be 'is at least seven hours' because it's implied you need an additional hour to reach the minimum of 8 consecutive off-duty hours before you can lawfully and safely drive again.
Sleep deprivation is a significant factor that can impair your ability to drive safely. A study mentioned in the question demonstrates the importance of adequate rest by measuring the effects of total sleep deprivation on professional drivers. These drivers exhibited diminished performance on driving simulations after being deprived of sleep for 27 hours, as opposed to their performance after normal sleep. It is therefore critical for one's safety and adherence to health standards to ensure they have sufficient off-duty time before operating a vehicle.