If you can't close the roads (which is typically difficult for all but the largest races), I think you really need to communicate to the community what is going on so that they can prepare. For example, if you are in a subdivision, see if you can get some notification in their association newsletter. If you involve them, I think you'll find not only less traffic issues, but they will typicially come out and cheer and some will even race.
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Depends by what you mean by "police escorted but the roads are still open".
I've done a lot of 5Ks and 10Ks where the roads were partially open: a right hand lane was coned off for the runners, so traffic could run parallel to us but in their own separate lane. The police were stationed at cross streets and allowed cars to cross the race course only when there was a break in the runner stream. That works fine.
However, if you're talking about having the police escort just the lead pack and then the rest of us share the road with cars...you don't want that. I've done one 10K where that was the case and it was a major distraction.
@ 5K: Ontario Mills 5K, Ontario, CA, 25:17
New Balance Palm Springs 5K, Palm Springs, CA, 24:32
Angels Baseball Foundation 5K, Anaheim, CA, 24:24
@ 10K: LA Chinatown Firecracker 10K, Los Angeles, CA, 52:15
Great Race of Agoura - Old Agoura 10K, Agoura Hills, CA, 51:40
Fiesta Days Run, La Canada, CA, 49:57