A scenic drive quoted at three hours regularly consumes five. The gap is not poor estimation so much as a model that does not describe how these roads are actually driven.
Estimates are built from average speeds
Navigation systems assign each road segment an expected speed derived from its classification and from recorded traffic data across many vehicles.
On a motorway that estimate is accurate because traffic moves at a consistent speed for long periods with few interruptions.
On a mountain or coastal road the recorded average blends confident local drivers with cautious visitors and tourist traffic, producing a figure that matches neither.
Curvature costs more time than distance suggests
A road that climbs a valley wall through repeated hairpins covers several times the straight-line distance, and each turn requires deceleration and acceleration.
Fuel consumption and driver fatigue both rise sharply under this pattern, and fatigue is what turns a long drive into an unsafe one.
Gradient compounds the effect, since heavily loaded vehicles and older cars climb well below the posted limit and cannot be passed on a narrow road.
Stopping behaviour is not in the model
Scenic routes exist to be looked at, and drivers stop at viewpoints, villages and photographs far more often than they would on a functional road.
Each stop consumes far more than the time spent parked, because rejoining slow traffic and regaining rhythm takes several minutes.
A realistic plan treats the stops as the purpose and the driving as the connective tissue, which produces a very different schedule from the navigation estimate.
Seasonal and structural constraints intervene
High passes close in winter and reopen on dates that depend on snowfall, while single-track sections with passing places can back up behind one slow vehicle.
Roadworks on mountain roads frequently involve single-lane working with long waits, since there is nowhere to build a diversion.
Ferries and seasonal shuttles introduce fixed departure times that dominate the schedule, and missing one can cost hours rather than minutes.
Planning a day that works
A workable rule is to plan for well under the daily distance a motorway would allow and to fix the overnight stop early rather than driving until dark.
Driving the more exposed direction of a coastal road in the morning avoids both the heaviest traffic and low sun, which matters on unprotected drops.
Fuel and food should be taken when available rather than when needed, because the interval between services on these routes is long and unevenly spaced.