Best Route Calculations Sometimes Incorrect

I have found my nuvi 660 almost bang on target.....I have used it many times across states and have found it similar to the routes planed through Google or yahoo...

Are you set to "Shortest route" or "Fastest"?

If you are set on shortest, that is exactly what you get.

I am taking about a trip of 30 miles that is all highway or the alternative is 4 miles shorter but goes through routes that have a similar speed limit but lights. The Garmin will send me on that path and more often than not and it just does not end up being shorter time wise.

I notice this also because the time of arrival is usually correct when it is mostly a highway route but almost never correct when it is a route with lights.

My Gamin quite often gives a route that I know I don't want to take at that time of day, so I go the way I know is quicker and it adjusts along the way. I think the gps knows the speed limit and assumes you can drive that fast, with no considerations for traffic, lights or other things that might slow your progress…..

I've run into a similiar problem as you've described. If you know this is happening, just continue on the route you prefer and the unit will adjust to your new route. I guess this is one of those quirks in route calculations. Fastest always seems best from my experiences.

gusb

Then that is why you are sent the shortest not the fastest way. Change the setting to fastest...

Dear A,

My nüvi 650 is set at "Faster Time". When I leave for work early in the morning (little traffic), the unit always indicates that the trip will take ten minutes. The trip takes 16-20 minutes and is through the streets of Brooklyn. Coming home takes half an hour. This causes me to wonder whether my unit may not be good at calculating driving time on city streets.

david

To the OP, it's important to understand that the GPS unit doesn't "know" anything about traffic (notwithstanding units using a traffic receiver) or lights. So it won't take these into consideration at all when planning a route.

The information in the database contains average traveled speeds of different classes of road types.

It uses this information along with the preferences that the user has chosen to attempt to calculate a mathematical solution corresponding to those parameters.

It is not designed to give the "best" route only the fastest time or shortest distance in accordance with the data it has.

The map data comes from Navteq not from Garmin for the most part, however, since traffic and lights are constantly variable, it's unlikely for them to be included in the map data for individual street segments.

I don't "wonder" about it at all. There are too many unknowns involved. It is not just "your unit" but all of them....unless maybe you have "live traffic" capability.

Some brands may make different (better?) assumptions about city streets but none are really accurate.

Do any of them "adjust" the arrival time as the trip progresses? That is, if the original estimate was 30 minutes and 15 minutes later, you have covered less than 1/4 of the trip, does it UP the ETA.....by more than the remaining 15 minutes ??

I've never been in that situation to test mine.

This exact problem is being addressed in a GPS unit that will be coming out in the near future. Each unit will share data with the a sattelite so you will know exactly what the drive time is through an area. The more units in that area the more accurate the info. I cant wait for it to come out and everyone to get one. As opposed to the unit telling you there is traffic you will know exactly how slow it is moving.

Dear DT,

The ETA does adjust during my ride to work. As I warm the engine in front of my house and look at the GPSr, I know to add ten minutes to the ETA so that I know I have time to reach work without rushing. I have adapted to the unit's idiosyncrasies.

david

If you know that the POSTED Travel Time on the GPS is off because of unknown traffic problems. Like actual speed or red Lights or traffic itself, just make it a habit to give you enough time for these road blocks.

The GPS is only a tool, to be used as a tool. If you know that the driving time is going to be off, then adjust the start time accordingly.

I drive at least once a week from Cecilia Ky to Louisville Ky, about 54 miles. I don't like using I65 because of the truck traffic and accidents. So I travel US31w. I have about 13 traffic lights for the first 30 miles and at least 25 within Louisville and depending on the time of day there could be a lot of going to/from traffic. The ETA is only off about 3 to 5 minutes most of the time. Sometimes I even beat the ETA by a few minutes.

Sure you might even show up at your destination a few minutes early. But would that really hurt? You might even not miss a flight!

It is just like back in the days when I was still working. Fellow employees would time there drive to work. Say they could make the trip from home to work in 35 Min in good driving conditions. On bad weather days they would always be late and use the excuse the weather caused them to be LATE.

NOT TRUE! You have to anticipate some problems and adjust accordingly.

I know someone out there will say well what about the time that HE/SHE was stuck on the freeway for 4 hours because of an accident just ahead of me and I had no way to get off.

You can't anticipate everything all the time. So some excuses are valid.

But waking up in the morning with snow on the ground and still leaving exactly 35 minuets is not exactly using common sense.

Reading H Hannah's great reply reminded me of an email my wife received a couple months back. Here it is:

My parents told me about Common Sense early in my life and told me I would do well to call on her when making decisions. It seems she was always around my early years but less and less as time passed by.

If not, join the majority and do nothing.

My favorite anonymous quote: "Common sense is an oxymoron."

It's hard to calculate traffic lights into the equation. Don't really know if your going to hit a redlight or not. I figure in about 4-5 extra minutes onto the time if the driving distance is 5-8 miles in the city. It will be cool when the future garmin units relate travel info to each other to let you know of traffic slow downs better than the current MSN info.

It would be nice to have a third choice of routes. Make an algorithm that would select an optimum route if it saved more than x% of the total miles if it costs less than y% increase in time. I recently plotted a route using faster that was 55 miles and 42 minutes. By forcing a via point I had the Garmin recalculate my normal route which came out to be 45 miles and 43 minutes. I will gladly take one more minute to save 10 miles.

My options for a trip tomorrow are:
Faster - 79 miles and 1:29
Shorter - 66 miles and 1:34

So I will take the shorter route but there may be better routes. Just don't know.