$[$Route Choice$]$ Now, consider only those auto trips. There are two routes

connecting Rosslyn and Foggy Bottom with performance functions

$t_1=5+0\mathrm{.}003x_1$

$t_2=7+0\mathrm{.}001x_2$

with $t\text{'}$s in minutes and $x^{\text{'}}\text{'}$s in vehicles per hour.

a$)$ What are the route flows and the total travel time under user equilibrium?

b$)$ What are the route flows and the total travel time under system optimum?

Can you propose a toll scheme under which the tolled user equilibrium route