Anyone know if the 09 TL's nav unit calculates the time to your destination based on your speed and speed limits? My Maxima had a setting where you said "highway speeds are x mph", "residential streets are y mph" and it would use those static numbers in calculating when you'd arrive. It was really not that accurate since traffic could be an issue.

I think some of the TomTom's actually use your speed and the speed limits of the roads (if known) to constantly recalculate when you'll arrive making it much more accurate.

Any ideas how the TL works?

Jared

 

Not sure, but it's less accurate then the Eclipse system in my Lexus.

 

The destination is based on miles driven. Say if you are stuck in traffic, and your speed is 10mph, it will calculate as if you will drive 10mph all the way but as soon as you start moving again and say do 50mph, the Navi will now show you the time to destination at that speed all the way. Mine is usually accurate -5 minutes or I usually get there 5 minutes earlier than indicated on dash.

 

I don't think that's how it's calculated. It is based on road miles but it will use a constant formula based on the average speed of the roads involved (best case scenario) or the road's speed limit (less then ideal). This is how the nav system chooses which route is fastest. There are plenty of times where the system 'thinks' a road is faster or slower then it actual is which causes errors.

For example, if you are traveling 20 miles, 10 of those miles are at 20mph and 10 of those miles are at 40mph it will use this formula

10 miles @ 20 miles per hour = 1/2 hour or 30 minutes
10 miles @ 40 miles per hour = 1/4 hour or 15 minutes.

Total trip = 45 minutes.

Alternatively there may be a route to the same destination that is 30 miles but you're on highway most of the way

30 miles @ 60 miles per hour = 3/6 hour or 30 minutes

Total trip = 30 minutes.

So the Nav system picks the second option because it's faster.

If you leave at 12pm you should arrive at 12:30pm.

However if you instantly get stuck in traffic for 8 minutes you will notice that the time to arrival is still 30 minutes, but it's now 12:08pm so your arrival time is now 12:38pm. If it took into account your current speed (say 3 mph) it would recalculate your time as

30 miles @ 3 miles per hour = 30/3 hour or 10 hours. I've never seen the system increase my destination time (though with up to date traffic it's certainly possible)

So the problem is that traffic**, stop lights, actually speed are generally not taken into account (at least on the Eclipse system). The great thing about the Eclipse (and I guess the Maxima) system is you can adjust the mph for different roads. I remember adjusting residential streets and main roads down 10mph from default and increasing hwy to 65mph which gave me a much more accurate reading.

**unless you have accurate traffic warnings and reroute traffic selected.

 

interesting - thanks for the info.

 

Somewhat related:

This oddity is in the 3G Navi; not sure about the 4G:

When searching/selecitng a destination using the "Sort by Distance" option, the POI's are sorted based on Straight-Line distance (i.e. as the Crow flies).

BUT the route is based on road distance.

In some rare/odd cases, you can choose a store/POI that is, say, 5 miles in the selection list, but the route is, say, 15 miles.

 

I think most systems work this way. It takes a bit more CPU power to determine an actual route vs. determing distance by straight line. I'm not sure which the 4g does but my Eclipse and my wife's Garmin do it like the 3g

 

4G doesn't do the straight line estimate. It shows the actual road distance.

 

Just tried it and yes the 4G does it seems like it's road distance but it is a little screwy. When I chose a distance far away it reported distance was 166miles but then when I selected it, the distance changed to 155miles. (this could be traffic rerouting at work).

So I decided to open the navigation PDF.

"When you select a destination, the
distance to the destination is the actual
road distance of the calculated route,
and not “as the crow fl ies.” When
viewing a list of destinations (points
of interest), such as restaurants, the
distance is “as the crow flies”

So I guess we have a little of both. It's too bad the POI aren't actual road distance because if your driving on a highway, there can be a HUGE difference between crow flies and road distance.

 

I feel like I am reading a S.A.T. question...lol

 

Had a case yesterday that seems to indicate that the traffic/congestion is used in the calculation. Made a trip I have made multiple times, and I know the estimated time usually shown by the nav. Yesterday it showed a longer time, with some significant traffic delays near the destination - shown by the orange and red route highlights.
Also, it seems that when I first enter an address, it shows the miles as the crow flies until it completes the route calculation, then it shows road miles.

 

I did a long trip (Boston to Philly) this weekend. The nav definitely recalculates for traffic as I did see my times sometimes increase. It also tells you when it's changing the route. One time it changed my route and then decided the other way was faster. This took me a total of 20miles out of the way (since there were no exits). Needless to say I was less then enthused.

Overall I'd have to say that the traffic function SUCKS! Anytime we hit traffic, I'd take a quick look at Google maps on the iPhone which showed the traffic congestion, but it didn't appear on the TL most of the time. I'm not sure why Google would update traffic info faster. I would think they're both using the same source.