I'm with MarkE - if the journey takes twice as long, you need twice the number of buses to stay on timetable. For example: Journey A to B, & back again - Normal traffic Bus 1 leaves A at 10am, arrives B at 11am, Leaves B at 11.10am arrives A at 12:10pm Bus 2 leaves A at 10.10am, arrives B at 11.10am, Leaves B at 11.20am arrives A at 12:20pm However, if roadworks double the time the route takes from 1 to 2 hours, even it buses leave A on time, they come back much later, as follows: Bus 1 leaves A at 10am, arrives B at 12noon, Leaves B at 12.10pm arrives A at 14:10pm Bus 1 leaves A at 10.10am, arrives B at 12.10pm, Leaves B at 12.20pm arrives A at 14:20pm