The four things that dry a road
Warmth. Warm tarmac evaporates water far faster than cold tarmac. A road at 30 °C dries in a fraction of the time of one near freezing.
Sun. Direct sunshine is the strongest drying force there is, and it is completely absent at night. The same road dries far more slowly after dark.
Wind. Moving air carries the water vapour away instead of letting it hang over the road.
Humidity. Air that is already saturated cannot take up more water, so on a humid, misty day the road stays damp for hours.
What that means in practice
A summer shower on a hot, sunny, breezy afternoon: the road can be dry again in well under an hour. The same shower at dusk in October, with no wind and humid air: the road may still be wet the next morning.
Shaded roads — forests, valleys, the north side of a hill, under bridges — dry last, sometimes by hours. A forest road can still be greasy long after the open road outside it is dry. In autumn, add wet leaves, which no forecast models.
How RideWindow estimates it
RideWindow's grip estimate does not use a fixed drying time. It looks back through the hours since the rain and works out how wet the road still is from the temperature, sunshine, wind and humidity after the rain. That is how the same "three hours since rain" can read as a dry road in July and a wet one in November.
The estimate is for clean, well-maintained asphalt at the forecast point. Shade, leaves and a poor surface are for you to judge on the day. Check your own area, or a city or mountain pass.