From my understanding, you are pretty safe as long as you don't provoke them (walking through the middle of them might be considered provoking) or near their calves. This article from the UK states "Where recorded, 91% of HSE reported fatalities on the public were caused by cows with calves". Basically, mothers with a child are going to be very protective.
Cows are a domesticated creature, so they are generally docile, but I would exercise caution because if need be they will use their mass and strength against you. I've heard of stories of farmers running from cows and narrowly escaping under a fence. Most of these did involve a farmer trying to separate a calve from it's mother. I've also heard stories of cows jumping fences.