If this were a highway, you'd have to cut the speed limit in half as people try to merge and possibly shift lanes after making a u-turn and only have a very short distance to get back up to speed. This kind of design looks like it'd only work at very low speeds or density, at which point you might as well just use a roundabout.

Edit: also it looks like you need about 6 lanes of space (2 in either direction and 4 for turning around, including an extra wide radius so you can go faster then 10 mph) all to effectively support 2 actual lanes of traffic.