Possible Clues:

• Warning sign that's self-evidently untrue?
• Maintenance operations that obstruct traffic
• Way factory gives reason for stop-go procedure
• Jam makers?
• M25, say, has mechanism for carriageway repairs
• Slower to progress, maybe reading new word in classic
• Training's a possible cause of congestion
• Pieces by strolling players resurfacing, maybe?
• Street repairs
• Highway deeds holding up travellers