The permittee must carry on the work authorized by the permit in such a manner as to cause a minimum of traffic flow interference. On main thoroughfares and in the congested districts there must be, at all times, sufficient traffic lanes open to permit a substantially normal flow of traffic. Unless this can be accomplished, it will be necessary that the work be confined to the hours between 7:00 a.m. and 5:00 p.m. or between Midnight and 8:00 a.m. and that temporary bridges be constructed across the opening to permit free movement of both traffic and pedestrians.