Specifically it looks like the biggest increases are on urban principal arterial and and urban minor arterial. I suspect larger vehicle sizes impact the trend on a broad scale, but consideration should be kept for increasing demand for urban arterial roads, and widening existing ones as metro areas continue to sprawl at more local scales.

I'd be very interested to see the data on pedestrian deaths by road type contrasted to road infrastructure growth by type. The fact that Rural Principal arterial deaths practically disappeared around the same time that urban arterial deaths started significantly increasing could speak to a change in road classification or area designation between rural and suburban.

Specifically Rural Arterial deaths decreased by 229 from 2014 to 2015. In the same year urban principal arterial increased by 231 Urban Minor Arterial increased by 136, and rural minor collector increased by 155. While it doesn't account for the entirety of the increase, I think there's enough there to look at how urban arterial roads are designed.

Edit: Added specificity.