With congestion, the total flow of vehicles may not change (usually it increases at first and later decreases), but the vehicles are going slower and slower. After a certain point, the lane or BRT system becomes congested.
When engineers talk about the capacity of a road or a BRT system, they will give a capacity for an acceptable level of service, rather than for the maximum number of vehicles or customers that could pass through a road or a system. The term saturation is also used to characterize a roadway, and in particular, the degree to which traffic has reached the design capacity of the road. Saturation level of a station refers to the percentage of time that a vehicle stopping bay is occupied. Thus understanding the saturation level of a station is a basic starting point in achieving high capacities and high speeds (covered in the next chapter), and if there is a risk of station saturation it also becomes a primary concern of service planning (covered in this chapter).
As a result, for BRT, the station saturation problem is the critical issue that needs to be solved to maintain system speeds-not intersections or headways, which can be defined as the time between two vehicles offering a service, conveying exactly the same information as frequency. A traffic signal can handle far more vehicles passing through it over the course of an hour than a bus stop can process. Vehicles can platoon, in which many can follow very closely, one behind the next, with very high frequencies. In BRT systems, by contrast, the constraint on capacity is the BRT station. Rail system designers thus frequently try to add longer and longer trains, but on city streets eventually this runs up against block lengths. With rail systems, the constraint on capacity tends to be the frequency that the signaling system can handle.
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Michael Jordan, former professional basketball player, 1963– 6.3.1 BRT Stations and Saturation You can have all the physical ability in the world, but you still have to know the fundamentals. 6.3 Basic Service Planning Concepts When I was young, I had to learn the fundamentals of basketball.