Possible Clues:

• Checkpoint board arranged with added security feature
• Unusually broad forward creating obstruction, in a way
• Barrier to progress en route
• Place for a 'Bridge Out' sign
• Obstruction on highway to prevent escape of a fugitive
• Reason to slow down
• Jam is for The Cars at the bar
• Barrier out of line stopping back door moving
• Police barrier catches a bold criminal in possession of precious stone
• The Jam or The Police are responsible for this and it annoys those in The Cars?