I agree with one of the posters below: this is a population heatmap, exactly as identified by XKCD (https://xkcd.com/1138/). Florida, which you mention, is highlighted in red only because this is a small-scale map with poor density representation. Here is Florida with the exact same data, as plotted by me in R with a little more resolution:
A population map, in other words. As a very basic demonstration of the capabilities of R, this is OK, but as a visualization, this is 100% useless. You would need something like "percentage of expected fatalities for a given stretch of road, given the type of road" to actually have something interesting. And that takes a lot more work.