What typically happens when a new roadway opens or a congested route is widened is that travel times first fall, as expected; but soon the reduced travel times attract drivers who previously had used other routes, or who had commuted at off-peak times, or who had used public transit. Thus the improved roadway ends up with significantly more traffic than the old roadway, and the price (in travel time) of the increased capacity is bid back up near its previous level.
Added roadway capacity can lead to a welfare improvements, but it mostly comes in the form of fewer people shifting their schedules to beat rush-hour traffic or deigning to take public transit, and not through reductions in travel times. In actual fact, as we've seen throughout the era of American suburbanization, vast new roadways have probably substantially increased average travel times [1] by markedly changing patterns of land use (i.e., by giving rise to the modern low-density, car-dependent suburb), albeit while also engendering some welfare gains (e.g., increased suburban living space).
If you want to prevent induced demand from gobbling up expected improvements in travel times when you build new roads, you pretty much either have to (a) decrease demand by deliberately increasing the financial cost of travel (e.g., through tolls or congestion charges), or (b) over-build roadway capacity in the extreme, to the extent that just about everyone in the region can fit on the roads at once, alone in his or her own car (as is the situation in cities like mine).
0. https://en.m.wikipedia.org/wiki/Induced_demand
1. I mean in comparison to a counterfactual alternative history in which the urban-suburban commuter freeway systems typical of American cities went unbuilt. Commute times in European metropolises, which mostly lack the extensive commuter roadways and concomitant suburban sprawl of their American counterparts, are broadly lower than in American metros of similar sizes, for example.