A coastal drive routinely takes half again as long as the estimate. The discrepancy is systematic, and it comes from the way shoreline roads are built rather than from traffic.
The road follows the coast, not the journey
A shoreline route traces every headland and inlet because there was nowhere else to put it. Cutting across would have required bridges and tunnels that were never justified.
The result is a road whose length bears little relation to the straight-line distance between its endpoints, with the difference concentrated in the most scenic sections.
Route planners measure that full length correctly but apply an average speed drawn from roads with far gentler geometry, which is where the error begins.
Gradient and camber suppress speed
Coastal roads climb and descend constantly as they cross headlands, and the corners on those climbs are often tighter than the road's classification suggests.
Drivers slow for blind bends above a drop far more than they do for a comparable bend inland, and the cumulative effect over a day is substantial.
Heavy vehicles and caravans compound this, since a single slow vehicle on a road with no overtaking opportunities sets the pace for everything behind it.
Single-track sections change the arithmetic entirely
Where a coastal road narrows to one lane with passing places, progress depends on how often you meet oncoming traffic rather than on your own speed.
Each meeting requires one vehicle to stop, reverse if necessary, and wait. In season this can happen every few minutes on a popular stretch.
An average speed on such a road is a meaningless figure. The realistic planning unit is the number of miles, treated as if each one takes several minutes.
Stopping is not optional in practice
Coastal routes are lined with viewpoints, and the honest reason for taking the road is to look at what it passes. Nobody drives one without stopping.
Car parks at the best points fill early, which adds circling time, and the walk from an overflow area to the view adds more.
Planning a shoreline day around driving time alone guarantees either a rushed drive or a late arrival, and the second is more common.
Weather changes the road's character
Sea fog can reduce a headland road to walking pace with no warning, and it forms and clears over short distances along the same coastline.
Wind is the other factor, particularly on exposed causeways and bridges where crosswinds affect high-sided vehicles and occasionally close the crossing entirely.
Building slack into a coastal itinerary is not pessimism. It is the only way the day works when conditions turn, and it is why experienced drivers book accommodation short of their theoretical range.