The shortest distance between two points
The shortest distance between two points is under construction
This is a deconstruction of a common phrase. The original phrase is "The shortest distance between two points is a line".
As a side note, the original statement is true under the confines of euclidean geometry, however when you project a globe onto a map, the truth of it breaks down (anyone who has seen an airplane flight path that travels far north or south probably noticed this).
The humor in this joke lies in the fact that the original phrase has been modified to become a pessimistic statement about travel. The assumption that you will always travel slower to any given place than you could go is humorous.