Possible Clues:

• Drug carried by pregnant woman gets one to slow down
• Local examples of available drugs for slowing down joyriders
• Couple of examples of local drugs that will slow you down
• Traffic slower-downer
• Driver's slower
• Over which, e.g., Picasso would proceed cautiously
• Ridge in a parking lot
• Parking lot safety feature
• Career knock will slow your progress down
• Drug mule - the first discovered in back bar with sleeping policeman