According to the article, your theory is simple yet wrong:
> Here is what the frustrated safety experts will tell you: Americans are driving more than ever, more than residents of any other country. More of them than ever are living in cities and out in urban sprawl; a growing number of pedestrian fatalities occur on the fringes of cities, where high-volume, high-speed roads exist in close proximity to the places where people live, work, and shop. Speed limits have increased across the country over the past 20 years, despite robust evidence that even slight increases in speed dramatically increase the likelihood of killing pedestrians (car passengers, too – but the increase is not as steep, thanks to improvements in the design of car frames, airbags and seatbelts). American road engineers tend to assume people will speed, and so design roads to accommodate speeding; this, in turn, facilitates more speeding, which soon enough makes higher speed limits feel reasonable. And more Americans than ever are zipping around in SUVs and pickup trucks, which, thanks to their height, weight and shape are between two and three times more likely to kill people they hit. [emphasis added]
From other statistics I've seen, the massive increase in SUV's is mentioned as the single largest factor.
Indeed, the article directly refutes your theory:
> Ask a room full of safety experts about smartphones and you will get a mix of resignation, bemusement and contempt. “I tend not to buy the smartphone distraction stuff,” says Garrick, echoing nearly identical comments from just about everyone I talked to. “To me, it reads as shoving aside actually dealing with the relevant issues.” What particularly bothers him, he says, is how poorly thought out the distraction discourse tends to be. In the UK, Belgium, Germany, Spain, France, Austria and Iceland, for example, pedestrian deaths occur at a per capita rate roughly half of America’s, or lower. Are we really to believe that the citizens of these countries are 50% less susceptible than Americans to distraction, by their phones or anything else? ... “All this talk about pedestrian distraction, driver distraction? It’s such a distraction,” says Ben Welle of the World Resource Institute for Sustainable Cities. “It puts all the responsibility on individuals, and none on the environment they operate in.” [emphasis added]