Roads are widened all the time with the goal to increase average speeds. I can think of counter-example where it's more clear that travel time is not an indepe

Roads are widened all the time with the goal to increase average speeds. I can think of counter-example where it's more clear that travel time is not an independent variable to road width: - lanes on highways are extra wide so you drive faster. (The wider the lane, the faster people will drive, the margin of error is greatly reduced allowing a faster travel speed). If what you were saying is true, then there would be first a lot of projects to narrow lanes to the minimum in order to increase the number of travel lanes. IIRC, US highways have as a standard a 13 foot width (I might have that somewhat wrong), IIRC as well, the absolute minimum width is more like 9 feet. There could be almost 50% more lanes by narrowing, but that would reduce traffic speeds.