In reality, the metric probably needs to be a lot more complex than what I hacked together here. It's actually more like "don't turn left across traffic, left turns are ok onto one-way streets, etc", my metric was simply based on turn angle, and there are a bunch of scenarios where that falls down.
The rule probably works well in travelling salesman-like scenarios - you want to optimize the route to deliver multiple packages.
Having played with adjusting these kinds of metrics quite a bit, it's never as easy as you want :-(
I believe UPS has done this for years to streamline their delivery-truck routes, albeit probably not in real time.