Compute default price of anarchy.
2/1.5=4/3.
Why exclude toll revenue from social travel time?
It is modeled as a transfer, not physical delay.
Compute transfer social optimum.
The optimal shortcut flow is z=capacity(constant_time-overhead)-demand=1(1-0)-0.5=0.5, which already lies within [0,0.5], so z=0.5. The total social time is (0.5+0.5)^2/(2(1))+1(0.5-0.5)+0(0.5)=0.5.