Transfers are the big issue. Many if not most trips will require at least on transfer. When you show up at the station at a random time, you have to wait on ave

Transfers are the big issue. Many if not most trips will require at least on transfer. When you show up at the station at a random time, you have to wait on average 1/2 of the headway time, but if you miss your transfer, you have to wait the entire time. With buses and streetcars typically being delayed by traffic, you have to count on missing the transfer more often than not. If you have to make two transfers on a 15 minute headway, then count on adding 30 minutes of waiting to a trip that might have only taken 30 minutes total. There is a massive difference between a 5 minute headway and a 15 minute one.