Here's everything I know about the science of traffic (which, frankly, isn't much)<br><br>
Any given highway has a capacity. This capacity is the number* of cars that can safely share the road at the posted speed limit.<br><br>
If the number of cars on the road exceed that capacity, the speed will go down and the system will be highly susceptable to any perturbation.<br><br>
In a circumstance in which the traffic flow is near capacity, very small perturbations of the system can set off a jam.<br><br>
An example of a perturbation could be someone tapping their brakes. If car #1 hits hir brakes and slows down 5mph, then car #2 (behind car #1) has to tap their brakes and slow down ~10mph (because of the delay in reaction time, further slowing must occur to prevent hitting), Car #3 has to slow down 15mph... etc.<br><br>
On ramps and exchanges also have a capacity which may or may not be the same as the capacity of the road. On an exchange there's a limit to the number of cars that can get though (per hour) before it starts to get congested. Once the on ramp gets congested, it starts to slow down the main road, which if it is at or near its capacity, can cause futher jams.<br><br><br>
*technically capacity is a rate, so #cars/hour, not a precise number of cars.