Left Exit or Right Exit Based on Time of Day

You don't mention what type Garmin GPS you have but on some newer models there is a feature called TrafficTrends. If this feature is turned on, which I believe it is by default, the Nuvi may know from that data about the time sensitive Car Pool lanes.

It's also possible that the map data simply contains this information, although there are HOV lanes like this in Phoenix and I don't recall it routing me differently depending on time of day. FYI, I have TrafficTrends turned off.

When you say "This suggests that Garmin either doesn't know about the exit's Carpool status or does not care" maybe it's more that Garmin doesn't know if you have 2 or more people in your vehicle so it will always route you to the exit that isn't the HOV one during HOV hours. After HOV hours it will route you on the HOV exit if it's faster or shortest, depending on how the device is setup.

I would lean towards the traffic theory as well. Your GPS could think that one way is quicker at certain hours and give you that option.

I am using a nuvi 855, I don't believe it has this feature.

To clarify, I can set the Carpool Avoidance on or off. If I tell it to not avoid Carpool lanes during the Carpool only hours for the left ramp, why wouldn't it route me to take the left (HOV) ramp? The left exit is consistently faster at all times, it is also shorter in length.

Perhaps, but I've found the left exit to be faster at all times, and *much* faster during HOV hours, which is when my nuvi 855 routes me to take the slower non-HOV (right) exit.

You are running into the problem of just how much detail about an area is stored in the unit. Published reports originating from sources at NavTeq (and no, I don't have the articles) have stated in the past they provide all that information to companies that purchase their data. The amount of data they provide for just some major cities can be in the terabyte range. How much makes it into the map on your unit is the fine line Garmin has to walk.

So, the raw map data knows the HOV lanes, their exits, hours of operation and all the details, but they haven't been put into your map because of the amount of storage available is limited. In some areas, because of HOV restrictions roads may show but they are blocked from routes because the unit doesn't know if you meet the requirements or not. Making it a configurable option would add too much complexity to the program and the amount of map data would also have to increase to cover those times when you could use the lanes over when you couldn't. It's best described as one of those situations where local knowledge overrules the guess of a programmer many miles away.

My nuvi knows the HOV lanes in some cities, all those between Toronto and Florida Keys via I75. It does not know the hours of operation and does not navigate by time of day.
For some reason it does get screwed up and sometimes tells me to use the non-HOV exit. I've always assumed it's because its accuracy has degraded and thinks I'm in a different lane.

I'm confused by your post. You said that my GPS doesn't know about timed HOV restrictions, but as I said earlier, the GPS *does* know to only route the left exit after 9:30 AM. (time confirmed). It routes via the right exit all other times. The time matches the HOV restriction, but the carpool avoidance setting has no effect.

Then the problem with carpool lanes has been addressed. It was a problem in that there is a major highway in our area that is closed to all but carpools during the peak periods but open at all others. If you attempted to plot a route using that road, the unit wouldn't use it at any time, day or night. It always routed you over alternatives.

I've never tried routing using those particular lanes because I know the restrictions - but then I don't drive during peak periods either.

How is it addressed? During the carpool hours the nuvi always routes to the non-carpool (right) exit, even if you do not have it set to avoid carpools.

What highway is this? The only one I am familiar with that is entirely HOV during certain hours is Interstate 66 in Virginia/DC.

It's addressed in that you can now at least route over the carpool lanes. "Road Lock" is something Garmin uses to assume your current position is on the road they are routing you over. The "lock" may prevent the unit from knowing which lane or lanes you are actually in, so it assumes you are in the widest portion of the roadway.

And yes, it was 66 where the problem was reported. Using Garmin you could not route on 66 inside the beltway. Navteq allowed you to route over 66 so it wasn't a Navteq issue, it was how Garmin coded the data from Navteq before assembling it into a map package.

The Carpool routing doesn't necessarily have to mean that you are in a designated lane, but rather what exits are available. For example, if the carpool lane is blocked because of a sofa or something is in it, I may be in another lane but still eligible to take a carpool only exit.