Recently my Garmin advised heavy traffic ahead. I used the detour suggested & found when I rejoined the main route I was merging with vehicles I recognised which had not taken the detour. The detour route had more traffic than usual & hence the travel times were much slower than the posted speed limits making the detour no quicker than the congested main route. The lesson I learned was don't always trust the detour advice.
Ditto to terry_hannah. If you are going to a very unfamiliar area or taking a route you never had before, it's a good idea to print the directions. Just like Terry said, the alternate route may not be the best & construction is sometimes not always known by every site or service.