It takes a bike around 50 m to get to very near its maximum speed. Surely to have effective speed limiting, barriers would have to be every 100 m or less. Is that what is being done?
Oh gawd. If you ever ride from Remiremont up to the Col de Bussang stick to the road. The cycle path follows the old railway track and crosses around 50 roads in its 30k length. Each intersection is "protected" either side by overlapping half-barriers at gut height: these are made of tree-trunks about 8" in diameter, some of which have splintered ends.
It's a beautiful ride, but doing a double zig-zag every few hundred metres while watching for road traffic and and the dandering/dawdling populace is nerve-racking, and accelerating again every time is killing.
While I'm on the topic, a new cycle path near here has a red & white double barrier at the end. It's in the edge of the forest, and when the sun's shining the barrier as good as disappears in the dappled light. Two of my friends have hit it at full bore.