America's roads got slightly safer between 2016 and 2017, the federal government reported on Wednesday. In total, 37,133 people died in car crashes last year, compared with 37,806 who died the preceding year.

The welcome news comes after two consecutive years of significant increases; fewer than 33,000 people died on American roads in 2014. But the longer-term trend has been overwhelmingly positive. America suffered more than 50,000 highway deaths per year in the early 1970s.

That change is even more impressive when you consider the steady increase in miles traveled over the same period. Fatalities per 100 million vehicle miles traveled have fallen from 3.35 in 1975 to 1.16 in 2017.

The National Highway Traffic Safety Administration also released preliminary projections for the first half of 2018, and there the news was even better. An estimated 17,120 people died on the roads in early 2018, 3 percent fewer than died in the first half of 2017.

A number of factors have contributed to the long-term trend toward safer roads, including cars engineered for better crash-worthiness, increasing use of seatbelts and airbags, and stricter enforcement of drunk driving laws.

In the 1970s, 60 percent of traffic fatalities in the US involved alcohol use. In 2017, it was less than 30 percent. That's still far too large a number, of course, but the trend is in the right direction.

We are also beginning to see evidence that advanced driver assistance features like lane departure warnings, blind spot warnings, and automatic emergency braking prevent crashes and save lives. But these technologies are too new for researchers to quantify exactly how significant they are in practice. There's also a risk that drivers could become over-reliant on driver assistance technologies and stop paying attention to the road, offsetting some of the safety benefits.

Car and technology companies hope to deliver much more dramatic safety improvements in the next couple of decades with the development of fully self-driving cars. Drunk, tired, and distracted drivers account for a large fraction of roadway deaths. Hence, if software can drive as well as the average sober, alert human driver, its widespread use would save thousands of lives per year. And in the long run, companies hope to make self-driving systems that are much safer than even the best human drivers.