In all counties of the first class of this state which have police officers for the supervision and regulation of traffic on county roads therein, any member of the police force thereof who shall have honorably served on the force for a period of twenty years, shall have attained the age of sixty years and shall be found physically unfit for further services thereon, may be retired on half pay by the board or body having charge and control of the force, or upon his application shall be retired upon half pay.