Speeds used to calculate travel time

[Answered by John Cromartie] We had speed limit designations for all of the road segments. With the data that we used for this version, the speed limits were capped at 55 mph, which presented a problem. Clearly that over-estimated travel time in some areas where speed limits are higher than 55 mph. For that reason, and for other reasons, we did a series of tests against actual travel times using Google Maps. We clearly saw that the bias was towards over-estimating the time it took, because of that 55 mph cap. An adjustment was made to all of the travel times based on that random sample of segments we chose to compare our results with Google Maps.

We now have, for the next version (for the update for 2010), we have a more complete road network database that takes the speed limit up to 65 mph. And we also are going to include more local roads than we did before. So the answer to the question is, we have a range of speed limits, hopefully that accurately reflect the posted speed limits. So it doesn’t take into account traffic, commuting times, rush-hour, weather, etc. It’s the posted speed limit, as close as we can get with current data.