Design speed governs street character
Highways and arterials are engineered for speed, with wide lanes, long sight lines, and minimal cross-traffic. A highway designed for 60+ mph features grades no steeper than 3 percent and curves with radii measured in hundreds of feet. Arterials run 35-45 mph and accommodate some cross-streets and turning traffic.
Collectors are the transition zone, balancing movement and access at 25-35 mph. Local streets aim for 20-25 mph, prioritizing neighborhood access and pedestrian comfort. This cascading hierarchy means a single neighborhood block should never carry highway-speed traffic, yet many do where zoning has failed to prevent it.
Pedestrian experience and intersection logic
A 20-mph local street narrows sight lines, adds curb bulbouts, and features frequent stops - all of which increase driver attention and reduce collision severity. By contrast, a collector at 35 mph needs 200+ feet of stopping distance and minimal pedestrian crossing points. A woonerf (European neighborhood street) inverts the hierarchy entirely, granting equal priority to pedestrians, bicycles, and occasional cars, with the street itself paved continuously at walking grade.
Street typology enforcement is critical. Arterials widened to 8 lanes induce higher speeds; narrowing a local street from 36 feet to 28 feet has measurable effects on driver behavior and crash severity within months.