(31 Dec 2014, 7:03 pm)Dan wrote I'd suggest otherwise... Traffic flow does have trends, but is not completely consistent.
As I'm from Sunderland, I could provide quite a few examples in this city alone, before even thinking about examples from across the region. The Wearmouth Bridge is a good example which springs to mind; whilst you can guarantee that the bridge will be busier during peak times, how fast you get over the bridge depends on the drivers of the cars who are on it. During term time, I usually cross the bridge at about 08:00am each weekday morning (sometimes earlier, sometimes later). There is always traffic, but sometimes the driver can get through without stopping once. Other times, due to traffic congestion, the bus is stationary for a number of minutes.
Let's use the 35 as the example of our frequent bus service (every 10 minutes). Your posts would therefore assume that each of the 35 buses would be delayed by the same amount of minutes in this half-hourly period. This is not the case. The 08:00am bus may be delayed by 5 minutes as a result of this traffic congestion, and the 08:10am bus may get straight through. I don't think it would be wrong to suggest that most people would aim to be at the bus stop around 5 minutes before their bus is due, so in theory, the 08:00am bus starts to pick up a greater amount of passengers than it ordinarily would. Further delays are created by this, and the process goes on until the 08:00am and 08:10am buses are running together. By comparison, the 29 runs every 20 minutes. Let's imagine that it is also due to cross the Wearmouth Bridge at 08:00am, and suffers the exact same delay as the 08:00am 35 bus as they're running together. It has more of a chance of making up time because it's a 20-minute frequency service, and passengers aren't going to go out to catch it 15 minutes in advance...
I agree with Dan here - my experience on our service 15 (every 7 minutes) is that it is very easy once you run late to start getting knocked back until the next one catches up. The effect usually snowballs until you end up with 3, and on bad days 4, running together! They can be slack as anything for time until you get to market day when suddenly all the time seems to disappear as the loadings go through the roof.