GPS differs on route for return trip; why?

Took a trip this weekend. I knew how to get to the general area I was heading, but could think of two possible ways to go. Wanting the “best route” and unsure of the exact location of the address, I used my Garmin.

When I began my return trip, I used the Garmin again, but this time it had me go the other route.:confused: 'Sup with that? There was no reason not to return home by reversing the original direction; no construction, etc.

Also, how is ETA determined? I drove through 15 mph towns with long red lights, as well as on a 65 mph highway. The Garmin predicted my ETA accurately.

Sometimes the layouts of exits, onramps and the fact that you’re on the outside of a wide road vs the inside changes the return distance, even on the same ‘road’.
That would apply more if you were on an 8-lane divided interestate than on a 2-lane undivided rural highway.
Some GPS can check or can model traffic, as well. It might have just guessed that traffic would have stunk one way but not the other.

(Oops, double-post)

My best guess is that the two routes were very close in time and/or distance to begin with. The difference in a particular set of exit ramps or an extra couple of blocks down a one-way street could have been enough to tip the balance to the other route on the return.

Not sure about the ETA question, but there’s probably more than one formula in use on the various brands and models. Mine (also Garmin) seems to be linked solely to the speed limit. When I get stopped at multiple red lights, I can watch the ETA get later and later. YMMV.

I’ve had the GPS tell me to go a different route in the same direction on different days, and the model I have does not do traffic. I assume it’s a function of when I put in the route and exactly where I am at that time, very possibly it scans near by roads to look for reasonable ‘exit route’, then just calculates from there.

Going in opposite directions I can see where the trips can vary, like on a trip where you are using mostly direct and efficient on-off ramps in one direction while on the return you have to use the looping clover leafs which do eat up time, possibly enough to justify another route - some clover leafs are very long. This may apply for calculating left vs right turns as well, I don’t know how the calculate it.

Another little GPS quirk is sometimes I know a shortcut that the GPS does not, when I take it it recalculates and shows that the short cut was in fact shorter and gives a earlier arrival time. It is evident that either the GPS doesn’t scan every route possible, or perhaps keeps one on better roads for convenience (or for the conspiracy theorist to keep you out of better neighborhoodsor pervent local roads from becoming heavily trafficked), but it would be nice if the GPS could scan for shortcuts in the background once the route is established.

I have a Garmin. It will recommend the fastest route and sometimes the difference is very small, so as mentioned earlier going a route in a different direction could push it over to the other route.

My Garmin is pretty accurate about predicting arrival time but it assumes things like driving over the speed limit (which is true for interstate driving) and does not take red lights into account. When I had the traffic option (which I dropped), it also took traffic into account for time estimating and routing.

Very cool responses! During the drive I was (jokingly) assuming more of a tail wind on my return trip.

There is that situation for the trip between Madrid and Seville. M->S takes you down Despeñaperros Pass; S->M goes via Mérida, which is actually a bit longer. Driving down Despeñaperros and picturing the drive up that same canyon is enough to understand the difference: you go down with your foot on the brake, up praying that your car won’t stall (in more serious terms, the speed going up is much lower than going down).

Don’t remember the brand. My son in law’s GPS ran us in a rest area and around back to a locked gate. If we could have gotten through the gate, it would have cut off some distance.

Most (all?) consumer GPS units don’t calculate every possible route between two points because it would take too long. If a GPS took 2 minutes to calculate a route it would get bad reviews and nobody would buy it. Instead, GPS designers sacrifice accuracy for speed of calculation. This leads to cases where the GPS selects the “wrong” road. Our Garmin always selects I95 N to I93 S instead of I95 S when we go anywhere south of Boston even though I95 S is “obviously” the better way to go. It simply doesn’t consider I95 S because I95 N to I93 S is good enough.

Remember, GPS units are designed for people who don’t know the best route to take, so selecting a route that adds 3 minutes to a 45-minute trip doesn’t matter much. At least, that’s the design philosophy. The reality is that people often use the GPS when they really only need guidance on the end of the trip. If you know better than the GPS, ignore it until you need really it.

Ditto. I tell everyone who asks that the GPS is designed to give you a good route, and that you should not expect the best route, as the calculations would take just too long.

Back to the several posters who mentioned traffic lights: My Garmin is about 4 years old, and I’m looking for a new GPS (any brand will do). I noticed long ago that the ETA is great on the highway, but then it slips minute by minute as soon as I get into the traffic lights and stop signs.

Here’s my GQ question: Does anyone know of a new model that does take traffic lights and stop signs into account? Of course, there’s no way that the GPS can know whether the light will be red when you get there, but if they would take some average arbitrary figure, it would make the ETA much more accurate. These computers are smart enough and fast enough to even add more time for left turns, and less time for right turns. All they really need is a database that includes info on where the lights and signs are located. Does anyone know if such a GPS exists?

And while we’re talking about my wish list, does anyone know of any GPS that evaluates the various routes using real-time traffic information, such as is on Google Maps? My 4-year-old Garmin supposedly does traffic, but the truth is that its info is limited to specific pre-announced construction projects. I find it frustrating when I’ve been on the road for 1/2 hour, and the traffic could be totally different than when I looked at Google Maps Traffic before getting in the car. (Not that I find Google Maps Traffic to be as accurate as I’d like, but it’s better than nothing.)

I’d pay a reasonable subscription fee if there would be a GPS which would interrupt my drive to say something like, “We’ve recalculated, and you’d better get of the highway at the next exit. There’s some pretty bad traffic ahead, but I’ll take you around it.”

The best route is subjective. Not a GPS, but Mapquest. Our of the goodness of her heart, a lady drove a visually impaired lady to a dog guide school where we gathered for training. No good deed goes unpunished. Nobody agrees on the best way there. Some like a route with some 2 lane and going through a small city. I opt for all interstate including some KamiKaze 4-6 lane urban freeways. Map quest not only subjected her to most of my route, but took a ‘‘short cut’’ with many extra turns near the end. It is dark, in a strange city, and your traveling companion can’t read your Mapquest print out, a map, or street signs. Now it it had been printed out in Braile…

She was asking for help on the return Sunday morning. I pointed to the highway across the fence from the hotel parking lot. Go a few miles East on it, South on the interstate, and then West on the urban freeway. That cut out about half the turns on the Mapquest route.

Both Garmin and Tom Tom have newer models that collect user route, time, and speed data and aggregate it. So if the speed limit on Maple is 45 mph but the street lights are out of synch and the actual average on a weekday afternoon is 30 mph, and enough Garmin or Tom Tom users drive it and plug their GPS’s into their computers for updates, the updated GPS’s will use 30 mph for routing and eta on weekday afternoons instead of 45 mph. Garmin calls this feature Traffic Trends and Tom Tom calls it IQ Routes.

I know that’s not exactly what you asked, but if enough drivers use them these features would take into account lights, signs and ordinary traffic too (for traffic incidents, you’ll still need the radio service).

Sounds good. What do they call this feature, so that I can look for it in the ads? Thanks!


“Garmin calls this feature Traffic Trends and Tom Tom calls it IQ Routes.”

From the post above :wink:

:smack: thanks! :smack:

My new Garmin Nuvi 1490 has a traffic receiver. I got it with free lifetime maps and traffic. It seems to know about traffic ahead and tells me how long the expected delay is. There is a detour feature to go around the delay but I haven’t played with it yet.

slight hijack…

I have 3 vehicles and 1 Garmin. Where can I get 2 more of the round plastic disks that the suction cup attaches to? No store here will or can sell me one; they want me to buy ANOTHER unit!

Thanks, CedricR.

The Garmin store.