First, the humans were not modelled as a fluid here but as greedy, selfish beings. Assuming no one (else) takes the new road, I would go for it giving me a 7.5 + 7 + 7.5 = 21.5 minute travel time – faster than that of anybody else. Soon most would follow me causing the two bridges to get more congested. This would also slow down those who did not change their behaviour. Now everyone would have a 33 minute travel time.
And, there are several real world examples of this happening in real cities. One "too good" road can sometimes cause severe imbalance in the city road network, which will slow everybody, even those not using that road.
PS. Using this model, no one would take your 100 mile bypass. It would not affect the system at all.