Freeways are, in principle, much safer than roads with at-grade crossings. With postwar design standards, they eliminate the frictions that are responsible to a vast majority of accidents: grade crossings, left turns, opposite traffic (since they have medians by design), and so on. They also maintain higher design speeds and capacity than less safe local streets. But a more interesting question for policy purposes than “are freeways safer?” is “does the construction of freeways increase road safety?”
For some evidence that the answer is no, see PDF-page 3 of a John Adams paper from 1987 arguing for the continued primacy of Smeed’s Law. Traffic deaths per unit of vehicle distance driven had declined in both the US and UK at a rate following a multi-decade log-linear trend: 3.3% per year in the US, 4.7% in the UK. Regardless of whether Adams’ theory is correct, we can compare actual death rates to the trendline to see what happened. In the US, where the data goes farther back, the greatest period of freeway construction started in the mid-1950s and ended in about 1970; this was also a period in which traffic deaths increased, even more than the trendline based on the explosive growth in driving predicts. Of course the Interstate system also led to traffic growth on at-grade arterials, but the greatest construction growth was in freeways, and on top of this suburban sprawl meant more people would be driving on both the new freeways and the older parkways.
The Smeed’s Law explanation of this is as follows: drivers compensate for the greater safety of freeways by driving more carelessly, on both the freeways and the connecting local roads. The freeways are still safer, but the presence of any safety-improving technology will translate entirely to higher speed and capacity (i.e. drivers keep less distance than they would otherwise), and more careless driving.
There may be other explanations out there – for example, the construction of more roads will cause more dangerous vehicles to start circulating that would not otherwise. These include heavy trucks, and also cars piloted by poor drivers who would not have driven if the construction of an expansive highway had redirected development in such a way that more driving would be needed.
But in either case, what this means is that even though a freeway upgrade of a notoriously unsafe road will make it safer, it will not make the overall road network safer. To argue by analogy with congestion pricing, it is possible that the only way to bend the curve and accelerate the downward trend of vehicle deaths, beyond reducing driving, is to make it more expensive to drive unsafely. For example, insurance requirements could be raised from $25,000 to the rough insurance value of human life in the US, which is in the millions. (The same should be true of any transportation system, but buses and trains are much safer for their passengers than cars.)