Average Speed vs. Posted Speed Limit: Which to Use for an Estimate
The posted speed limit represents a maximum legal speed under ideal conditions, not a realistic sustained average across an entire trip — a more realistic average speed accounts for slower sections (through towns, in traffic, at intersections), stops, and the simple fact that few drivers maintain the exact speed limit continuously for an entire trip, which is why using the posted limit directly tends to produce an overly optimistic estimate.
This is one of the most common specific mistakes in a manual travel time estimate, and it's worth understanding exactly why the posted limit isn't the right number to use directly.
Why the speed limit is a ceiling, not an average
A speed limit defines the maximum legal speed for a road segment under good conditions — it doesn't account for intersections, lower-speed sections through towns or construction zones, traffic slowdowns, or simply the reality that drivers don't maintain the exact maximum limit for the entire duration of most trips.
A more realistic approach
Using a realistic average speed for the specific route — informed by actual experience, a navigation app's traffic-aware estimate, or a reasonable discount below the posted limit for mixed driving conditions — produces a meaningfully more accurate estimate than using the maximum legal speed as if it were a sustained average.
How much lower a realistic average tends to be
The gap between posted limit and realistic average varies significantly by route type — a long highway stretch with minimal traffic might see an average close to the posted limit, while a route through multiple towns, intersections, or congested areas can see a realistic average meaningfully below the highest posted limit along the route.