The way it was explained to me was that drivers have a harder time estimating the speed of fast-moving objects that are differently sized than an automobile, such as bicycles and trains.
Meanwhile, bicycles that go with traffic will (relatively) approach the driver at a much slower speed, giving the driver more response time.
In my area, [these accidents](http://bicyclesafe.com/#wrongway) are way to common. I almost hit a biker like this the other day because they were riding against traffic past my apartment entrance.
[Here is a stack exchange question with some more stats on the topic](http://bicycles.stackexchange.com/questions/9033/why-is-it-s...)