The no-left-turns rule isn't crude, it's an incredibly valuable (and incredibly obvious) optimization. A mapping service that tells me to turn left in downtown San Francisco at rush hour is literally worse than useless. In many cases, if I follow its directions blindly, not only will I be delayed, I'll delay the traffic around me.

It's mind-boggling that Google, Apple, and other consumer-grade routing providers haven't gotten that particular clue yet. Seems clear that none of them have the vaguest concept of cost functions other than input from the local DOT traffic cameras.

From my house to the interstate, if you follow arterials, you take one left (at a protected light), followed by a right 6 blocks later and then the on ramp. Usually this is the route Google recommends. However during times of heavy traffic Google attempts to shield you from the horrors of waiting for two light cycles at the first intersection by instead directing you to take a left turn 2 blocks early, across traffic to a residential side street. You proceed for two blocks and turn right, then it expects you to do an acute blind left back onto the arterial street (with no light). This maneuver during times of heavy traffic is virtually impossible. The best part is that Google will invariably suggest this route to multiple drivers, so there is always a small line of suckers stacked up at the impossible left, unable to move forward or even back since it's a one lane residential road.

It's crude because it's too general. Making three right turns might be the right choice on a busy street in SF, but on a side street in a quiet suburb, making a single left is normally the better choice.

It isn't just the wait time, but safety. There is less to look out for when turning right vs turning left, resulting in fewer accidents. This is the same reason some employers require employees to back into a parking space when possible.

For me it would also need to know the date because it is reasonable to make a left turn in winter but not in summer.

As I pointed out elsewhere, that's what user preference settings are for, but I'm curious: why the seasonal difference? Snow routes?

Sure. So add a slider to the configuration page: "Avoid left turns when practical."

UPS truck also carries a hell of a lot more than your average employee is going to put in his car and on delivery-dense routes MPG isn't a great signal for efficiency

UPS trucks carry lots of packages so that they don't have to reload at the warehouse mid-day. That's not really a concern for Walmart employees, since they wouldn't be making any special trips to the store to load up on packages.

True, UPS has the advantage of denser delivery routes. But I'm sure Walmart has done the analysis and determined that they can deliver (some) packages more cost-efficiently than UPS despite this. Same with Amazon, which has been hiring their own couriers in some areas.

not many consumer cars carry as much weight as a ups truck. I am not sure looking at their mpg tells the whole story.

Weight is dwarfed by aerodynamics. These get shitty MPG because they are shitty technology, made possible only by lax emissions controls and fossil fuel subsidies.