To me, it's not just the road but also the conditions.
For example, 400 series are relatively low risk when traffic is light and flowing, but slow things to 80ish and the danger from other drivers goes up exponentially. Speeds are still high enough to be easily fatal, but now you have cranky drivers being unpredictable, impatient and aggressive. Speed differences between lanes, trucks missing you in a blind spot that you can't escape from quickly enough, lane hoppers, etc.
City streets have a high crash risk, but speeds are lower so fatalities less common. I personally love the cut and thrust of urban riding, but it's not for everyone. Backroads get dangerous at dusk with some animals and drivers whose eyes haven't yet adjusted. I'm extra cautious on suburban arterial roads, as speeds can creep to 80 easily, and other drivers are usually overly comfortable because of the open spaces.