Police presence is always part of Mass at some point or the other during the ride. Sometimes they help out and sometimes they hassle. Most of the time they don't so anything and are there to keep the peace if anything happens. A few Masses ago, pre-summer I think, there was an accident when we hit LSD, when a car bumped a Masser and everyone got all rowdy. Police were close by and came on the scene and got it sorted. Most of the Mass kept on moving as it's not supposed to be a confrontational sort of event.