I didn't read the parent as paranoid at all. I thought it was pretty cool.
Further, wrt the optimal/most direct route thing:
* the algorithm could be tweaked to "adpresence" en-route fairly easily
* "most direct route" is an ambiguous term. There could be two routes that involve almost same distance in miles, but the slightly longer one is likely to take less time. Which constitutes most direct? It turns into an optimization problem, possibly with no obviously optimal point (particularly when you account for fuzziness needed to deal with unexpected traffic patterns, accidents, etc).
* Finally, the ad-laden route could be a free ride, whereas the "direct" route could be paid. This may or may not get around regulations, but as long as the time delta was not significantly bad, I would choose the ad route.
There's a crucial difference between what you're proposing and what the parent suggests - user choice. At least as I read the parent, he's not saying you're given a choice of what route, etc. That seems like a crucial distinction to leave out if he meant that it would be at the request of the user.
There is a crucial difference also between what you are responding to, and what I said. I am not necessarily proposing the user choice should be followed or is even necessary. It is just a possible variation on the theme.
If you get in a cab and say "take me to $x" it is the cab driver's responsibility to take the best route. However, since metering is distance AND time based, there is a lot of wiggle room in what best route means. It is always an optimisation problem, and relies on imperfect data of the future conditions of the route. Further, in most american cities, there are many routes that are no more or less direct, as a result of the grid system. Therefore the car taking you on the route that goes past paid displays is not kidnapping. It is choosing 1 of n equivalent routes - the one most profitable for the operator, while still meeting the criteria of the user.
Further, even with advanced traffic and condition understanding available to a robotic car, the ability to choose optimal path will probably improve significantly, even when the small fudge factor to go past advertising is accounted for. So what if it is 30s less optimal on your ride to go past the advertised place, when the human cabbie would have chosen a route adding 5 minutes?
"If you get in a cab and say "take me to $x" it is the cab driver's responsibility to take the best route. "
Obviously this can vary by municipality, but in most cities drivers are required to take the "most direct" route, rather than the "best" route, if for no other reason than the latter would be so subjective as to be useless for enforcement purposes.
So if you're in a New York taxi (for example), the driver in theory isn't allowed to take you off the most direct route without your permission, even if another path would be faster.
I'm at (1,1) and I want to get to (4,4). Assuming a restrictive interpretation of most direct[1] and no one-way streets factored in[2], there are still two equally direct and valid paths:
(1,1) -> (1,4) -> (4,4)
and
(1,1) -> (4,1) -> (4,4)
Which is most direct? If the cab driver chooses to go with the later because his buddy pays him to go past the billboard at (3,1), what's to stop him? What's wrong with that?
[1] If most direct means "shortest path", then there are many more equivalent routes.
[2] One way streets also offer a nice choice of where to do turns around a block under favorable circumstances. Similarly road construction offers extra choices by detour.
Further, wrt the optimal/most direct route thing:
* the algorithm could be tweaked to "adpresence" en-route fairly easily
* "most direct route" is an ambiguous term. There could be two routes that involve almost same distance in miles, but the slightly longer one is likely to take less time. Which constitutes most direct? It turns into an optimization problem, possibly with no obviously optimal point (particularly when you account for fuzziness needed to deal with unexpected traffic patterns, accidents, etc).
* Finally, the ad-laden route could be a free ride, whereas the "direct" route could be paid. This may or may not get around regulations, but as long as the time delta was not significantly bad, I would choose the ad route.