gpserror
Sometimes I wonder, most cars do not have turning radius tight enough to make u-turns onto single lane roads.  While not "illegal" (i.e. no signs) I wonder if:
- target road has no left turn lane.  This would block traffic on primary roads and higher, causing traffic jams while a person waits for a gap.  Most of the time governments make these illegal due to the hassle for through traffic.
- target road is less than 2 lanes with a shoulder, or 3 lanes.    The u-turner would go "off road" when making a U-turn which implicitly illegal due to damage to the grass.  Of course doing a 3-point turn tends to be illegal on anything but a service road or a low traffic residential road...

These things are implicit and hard for a computer to know that these are problems drivers need to address but not available when the data does not include these issues with U-turns.