I'd also say that in urban settings, simply having a flashing light isn't enough. Don't feel that the cheapest light you can get will keep you out of trouble. You need to have enough light to compete with your surroundings. I say this because, last week a 28 year old woman in Portland was right hooked by a semi truck and died from her injuries. I noticed in the news video coverage of the collision that there was a small flashing light under the truck. You can see it in the second photo here (just in front of the tire).

Be safe out there folks.