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Perhaps to turn on something like 'Bicycles ahead' warning lights for an upcoming section with poor visibility, or a tunnel or similar.

Or a simple bicycle counter - authorities are interested to know how many cyclists are using this route and at what times of day etc.

Hmm, something like a chip timing station so you can time your climb?

Presumably there would be similar ones at the bottom and top also, and perhaps at other points along the way.

It is on the uphill side.

Aha, now I see.

Further guesses: Place to pull over and rest; Place for cyclists to make a U turn - perhaps just before a particularly steep or difficult section.

A pullout area to allow backed up vehicles to pass?

It's strange that it's on the downhill side and not the uphill, though!

Other guesses: Brake check area (stop and let brakes cool...); U-turn area as we sometimes see for the 'j-turn' configuration; The pullout is for uphill cyclists but they have to make a u-turn to use it.