The A61 takes traffic away from the old A61 route which went through the town so is effectively a by pass
It
relieves the traffic on
part of the old A61 through the town but cuts the town in two thus is a
relief road. A 'bypass', by definition,
passes by a town or village.
If you think the A61 is effectively a by pass, you must also feel that Toby Perkins is wrong in saying that Chesterfield needs one!