Not always the case, but you’re not wrong. Most of the times the new road or added lanes was needed because the traffic density had already increased. Kind of a chicken or the egg scenario. For a new road, well roads arent just built for no reason…obviously the road was needed, so now there will be traffic on it. Sometimes even just an influx of people using the new “alternative route” because they think no one will be on it from the old route, yet many other people had the same idea.
Exception to all this, however is evacuation routes. I grew up in the south, on the gulf of mexico. When hurricanes are coming and everyone is trying to leave, you need those huge highways. 30 years ago you would just have 1000s of people grid-locking 2 lane highways just trying to get anywhere away from the storm, and in some cases being stuck in their car for the storm. Now a lot of those highways are full-on 4 lanes with medians, huge shoulders, etc. These are everywhere across the south, more still being built. Even extra bridges built across bays and sounds that are largely unused (usually have high tolls). 99% of the time the big highways are mostly empty (which makes road trips super nice!) and someone not familiar would think it’s a huge waste. But come an emergency situation, and their purpose is served!