The police officer might have directed you to the diversion but only after you left, realised it involved a motorway. I personally would have come across the motorway, decided against using it, and gone back to the officer. I would have explained the situation and asked him what he recommends (and of course denied using the motorway option because I cherish my life).
Who knows? By the time you get to the officer, the road might be open again.
If it was that urgent, I might even consider cutting through some fields if necessary (but thats if I was desperate to get somewhere).