Cheaper gas prices have lead to increased driving which has lead to more gore on the roads. It turns out the relationship between miles driven and fatal crashes isn't just proportional, it's actually logarithmic.

In other words, 1 mile extra driven doesn't equal 1 more fatal accident or a ratio like that, it's more like 1 mile extra equals 1.4 extra deaths.

The reasons probably are complicated, but increased congestion and drivers spending more time on the road, leading to increased distracted and drowsy driving, at leading to increased fatalities. Congestion means your more likely to hit another car in your path then free-flowing, empty roads.

Long hours on the roads mean more drivers are distracted. 2/3rds of all distracted driver collisions (per NTSHA) are caused by day dreaming and paying attention to the scenery (or the guy in the clown suit at the big box retailer) rather then the road ahead.