The conclusion is basically that "smart highways" would result in improved traffic flow in and around cities. If you negate answer choice (A), you get: On "smart highways" there would [still] be the breakdowns of vehicles that currently cause traffic congestion. But that doesn't weaken the conclusion because the reason Eva thinks "smart highways" will improve traffic flow is that drivers will get more information about traffic congestion as it occurs, not that traffic congestion will no longer happen.
Answer B is wrong for a similar reason. We negate it to: Traffic lights, if coordinated by the system, would not assure a free flow of traffic. So what? That doesn't mean the situation won't improve. Eva's argument does not require that "smart highways" solve every traffic problem completely, just that "smart highways" would improve traffic flow.
Your mistake was likely one of not identifying the conclusion in specific enough terms. Try bracketing the conclusion and rereading it each time you check to see if a negated assumption attacks it.