My observation on roads is that it's a tradeoff between energy and time. Classical roads tend to follow the path of least work*time between two locations. That means that they don't go up steep slopes or across rivers unless they absolutely have to and will prefer straightish segments if possible. If your culture values time more than work then roads will invest lots of work in making roads that take less time to traverse (good pavement, straight segments, architecture that bypasses slow areas with bridges and road cuts, etc.) As the engineering capabilities of the civilization become more advenced, road networks get faster until some inflection point. When things get "fast enough" and folks start whining about "envionrmental issues" or there is a major technological shift such as instant transport devices then I would expect the road networks to stagnate. In a highly-advanced civilization there might be minimal road networks purely for the sake of entertainment.

One of the reasons that the interstates in the US are more straight than curved is (if I recall correctly) a requirement that one mile out of every three be straight so that they can serve as runways in time of war. The US interstates were designed by the military and for the military as infrastructure in case of war. Merchants then used them to reduce time to market and to destroy regional American culture. The same sort of thing happened in Roman times with the extension of the Roman road network.