(a) A local public entity is not liable under this Article for an injury caused by the adoption of a plan or design of a construction of, or an improvement to public property where the plan or design has been approved in advance of the construction or improvement by the legislative body of such entity or by some other body or employee exercising discretionary authority to give such approval or where such plan or design is prepared in conformity with standards previously so approved. The local public entity is liable, however, if after the execution of such plan or design it appears from its use that it has created a condition that it is not reasonably safe.
     (b) A public employee is not liable under this Article for an injury caused by the adoption of a plan or design of a construction of, or an improvement to public property.