i filter/split on the higways when traffic is going below 40mph. above that speed the spaces between cars increase and there is more of a danger of being cut off. also i don't split at an unsafe speed, ie 60 when traffic is going 15. i usually do about 15 to 20 mph faster than the slow traffic.
i have found that this annoys some riders and i get out of their way when a space opens up.
what the automotive crowd hates are the 75mph splitters. if you are doing that there is no possible way you can react to any obsticle in your way.
when in rush hour traffic i feel safer splitting than riding in line with traffic. i would rather be bumped on the side than be the meat in the sammich.