Estimating Trip Duration From Distance and Speed, and Why It's Always a Bit Optimistic
A basic travel time estimate (distance divided by average speed) assumes continuous travel at that average speed with no stops or delays — real trips almost always take somewhat longer due to traffic, rest stops, navigation adjustments, and other real-world factors a simple distance-and-speed calculation doesn't account for.
Understanding exactly what a basic calculation does and doesn't include explains why it's a useful starting estimate but consistently underestimates actual trip duration.
The basic calculation
A 240-mile trip at an assumed average speed of 60 mph gives an estimated travel time of 240 ÷ 60 = 4 hours — a clean, simple calculation that assumes constant motion at exactly that average speed for the entire distance, with no interruption.
What this simple calculation leaves out
Traffic congestion, rest stops, fuel stops, navigation detours, and simply not maintaining a perfectly constant average speed the entire way are all real factors a basic distance-divided-by-speed calculation doesn't include — each one adds real time beyond the clean theoretical estimate.
A more realistic approach
Adding a buffer to a basic estimate — commonly 10-20% for typical conditions, more for a longer trip likely to include rest stops, or for a route known to have unpredictable traffic — produces a more realistic expected arrival time than the clean theoretical calculation alone.