When determining a safe speed limit for a major street, there are two primary considerations:
How frequently potential conflicts arise on a given street
A conflict exists when a normal interaction, such as crossing the street while turning vehicles yield, is so close and at such a speed that a crash would happen unless sudden action is taken. In urban conditions, this is usually a factor of how separated modes are, and what the crossing demand is.
How active a street currently is or is expected to be
Crashes that cause fatalities or serious injuries are generally the result of conflicts happening at speeds that are too high for a human body to endure. Therefore, streets with a greater number of potentially serious conflicts and a higher level of activity should have lower speed limits.
The framework below summarizes a method for determining maximum safe speed limits based on the density of conflict points and level of activity on a major street. On urban streets where cities are required to conduct a study to determine the correct speed limit, they should use this framework instead of the passive 85th percentile speed study that the MUTCD recommends for highways.
The following provides thresholds for each activity and conflict density level, and apply these thresholds to example streets in North America.