18. Routing Engine Performance

All the bells and whistles aside, what really matters on any GPS is the quality of the maps, and how intelligently the GPS calculates the path from point A to B..

The most common GPS complaint I hear is "the GPS took me on a crazy route", followed closely by: "that definitely was not the best/shortest way."

The two most important components of any GPS navigation system are the routing engine and mapping accuracy. The routing engine is the software that lets the GPS decide which route it should choose to get from point A to point B. Many people falsely assume that simply having the same maps installed on two different GPS devices will result in the same routing choices. Not so, as there is a great deal of math, personal preferences, and economics involved when your GPS begins calculating the "best" route.

Not surprisingly, this also one of the most difficult and complex aspects for GPS manufacturers to get right, and is the aspect of GPS navigation that is least well understood by consumers. Nevertheless, routing engine quality should be at the top of any GPS buyer's checklist.

The two largest mapping data providers are NAVTEQ and Tele Atlas . NAVTEQ used to have the advantage in mapping quality in North America. However, Tele Atlas has recently improved its US database, and both companies are now roughly equal in quality. The mapping provider shouldn't be a determining factor in routing quality, as both NAVTEQ and Tele Atlas provide excellent maps. However, it IS important to consider how often a GPS manufacturer releases map updates. Garmin and TomTom release yearly map updates, while Magellan, LG, Navigon, and others release mapping updates on a much less regular basis. Virtually all GPS manufacturers charge a map update fee.

However, there is a lot more to a routing engine than just the mapping data provider. How a given GPS will determine the "best" route is not only the result of the mapping data, but is also the result of a given manufacturers decision on how many additional data points, or attributes, to factor into the calculation. For example, does the GPS know if certain roads closed after hours or on the weekend? That information is available to GPS manufacturers, but at an additional cost. Similarly, does the GPS know what the average rate of speed is for a given road? That information will also cost a manufacturer more to include.

Statistically speaking, more accidents happen when making left turns as opposed to right turns -- is the GPS unit factoring that into the route? Each company takes all this data and compiles it in different ways, making unique decisions about what data is more important than others. The number of attributes used varies depending on how much money the manufacturer is willing to spend on their mapping data. For example, a full NAVTEQ map database that includes all available attributes can cost the manufacturer twice as much as a basic set of attributes.

All Magellan and Garmin GPS units use NAVTEQ mapping data, while TomTom uses Tele Atlas.

To test the nüvi 500's routing engine, I entered three different trips of varying distance in three different regions of the United States and compared the results to a Magellan Maestro 4250 and a TomTom GO 930. All three test units had the latest available maps installed, and the most current firmware as of this writing.

Our first test took us along the Northeast, starting in Southbury, CT and ending in New York City:

Magellan's Maestro 4250, Garmin's nüvi 500, and TomTom's GO 930 all successfully get us to our destination address, but Garmin's nüvi 500 chooses a slightly faster route, provides more detailed navigation instructions, and is also the only GPS to announce what side of the street our destination address is on.

TomTom's GO 930 faired the worst in this particular test, routing us in such a way that the destination address is actually on the opposite (left) side of the street. We would then either have to make an illegal U-turn or go around the block to arrive at 135 Central Park West on our right. TomTom also has less detailed navigation prompts than the Magellan Maestro 4250. TomTom also uses yards instead of feet, which is less commonly used here in the United States.

Yahoo! Maps (not shown in the table above) chooses a route identical to Garmin's route, but estimates the trip will take 3 minutes longer (1 hour 22 minutes) than Garmin estimates. Google Maps also chooses the same route as the Garmin nüvi.

Our first test used a trip that was approximately 80 miles in length and took us from Connecticut to New York City.

For our second test, I started out at Fry's Electronics in San Jose, California and entered a destination address at 300 Grove Street in San Francisco (about 46 miles in distance):

Again Garmin chooses the best route, Magellan chooses the second best route, and TomTom's GO 930 chooses the least effective route. While Magellan and Garmin both route us along the 101 highway from San Jose to San Francisco, TomTom's GO 930 instead routes us along highway 880 North to 80 West -- an odd choice that adds around 4 miles (~6 minutes) to the journey, assuming there's no traffic 880 (which there usually is). Even worse, TomTom again has us arriving on the wrong side of the street, with our destination address on the left.

Both the Magellan Maestro 4250 and the Garmin nüvi 500 arrive with our destination address on the right side of the street, but only the Garmin actually announces us what side of the street our destination will be on. Occasionally the Magellan unit would verbally announce what side of the street the destination was on, but that was the exception and not the rule.

Garmin's maneuvering instructions are also by far the most detailed. It's not shown in the table above, but the angle of Garmin's turn icons actually vary to indicate the upcoming turn's actual angle, a feature not found on either the Maestro or the TomTom GO 930. Garmin wins routing test #2.

For the third test, I chose a short, 3-mile trip from Beacon Street in Boston to Cambridge, Massachusetts:

The results of test #3 are consistent with the previous two tests: TomTom's GO 930 chooses a route that's about 30% longer than Magellan's Maestro or Garmin's nüvi. Garmin chooses a route that's slightly longer in distance than Magellan's Maestro, but is actually a shorter drive time, and Magellan's Maestro plots an acceptable route that's only slightly worse than Garmin's. It's interesting to see such varied routing results even on a short trip of less than 3 miles; more proof that shows how very different each GPS manufacturer is.

Garmin edged out Magellan in all three tests, choosing slightly more efficient routes with more detailed turn instructions and shorter drive times.

TomTom's GO 930 unit performed least well in all three routing engine tests, producing the longest routes in all three tests, and putting us on the wrong side of the street at our destination address. Only Garmin's GPS unit announced what side of the street our destination address was on.

I've received many questions asking why it's important to arrive on the right side of the street. In more rural areas, it likely won't make much difference if you arrive with your destination on the right or left. However, when driving in cities along wide avenues, such as in New York City, it's very important to arrive on the same side of the street as your destination. Otherwise you'll need to make an illegal U-turn or drive around the block.

Regarding the estimated drive times, some readers have commented that these tests unfairly penalize TomTom for more conservatively estimating the drive time, and reward Garmin for unrealistically aggressive (short) drive times. I disagree with these assertions, as I have actually driven many routes with all three devices connected and compared the routing choices made by each. Further, only the Garmin nüvi "learns" from your own specific driving behavior, and adjusts estimated arrival times accordingly. That means that the longer you drive with a Garmin nüvi GPS, the more accurate the arrival times will become. TomTom and Magellan's devices, however, will always product the same estimated trip times.

All three tests show Garmin currently has the strongest routing engine. Magellan's Maestro unit performed admirably, coming in second place. TomTom's GO 930 (with TomTom's newest maps installed) came in 3rd place in all three routing tests.