Estimate detour time.
A detour costs time in three ways and only one of them is distance. The extra miles matter, but so does the lower average speed of the alternative route — a 40% longer road at 40 mph instead of 55 takes nearly twice as long, not 40% longer. The third cost is junctions: every set of signals or give-way on a diverted route adds roughly 30 to 60 seconds of deceleration, wait and acceleration, and a handful of them can outweigh the distance entirely. That is why a shorter back-road route through town frequently loses to a longer motorway run, and why comparing route distances alone is misleading.
Detour Time
Original minutes = miles x 60 / speed; detour minutes = detour miles x 60 / detour speed + stops x delay / 60; extra = detour - original
No, and often the reverse. Average speed dominates journey time, so a longer route on a fast road usually beats a shorter one through junctions and built-up areas.
About 30-60 seconds on average, allowing for the chance of arriving on green. Ten sets of lights is therefore five to ten minutes before any distance is considered.