@gtwilson This usually happens when there's a disconnect between posted speed limits and the design of the road. Traffic engineers will tell you that the 85th percentile speed is what the road should be posted at (i.e., the speed at or below which 85% of traffic is traveling in the absence of posted limits or enforcement). Artrficially low speed limits encourage people to ignore them, with predictable results--as was manifestly the case back in the days of the nationwide 55 mph limit.
Then again, some people just drive like idiots.