Possible Clues:

• Highway posting
• Reason to slow down
• Highway sign
• Unusually simple diet for drivers to keep to
• By the way, what is considered top rate?
• Driver's heads-up
• By the way there's a cap on the uppers
• Flashily ignoring this may gain you points - seventy, at most
• Sort of simple diet it's dangerous to exceed
• Bound to support amphetamine, though it restricts one's career