Historic Traffic Data - 2 Questions

ChrisHiggins

I'm having some trouble understanding the historical traffic instructions in network analyst, looking for clarity on two fronts.

The first - is a service area analysis in network analyst 'dynamic' when configured with historic traffic data?  By that I mean, if we have travel times measured at intervals of 15 minutes, and I compute a service area of 60 minutes, does that mean network analyst will look to the travel times at 8:00AM, 8:15, 8:30, etc. as the trip progresses over the 60 minutes?  Or does it just use a static measure of travel times at 8AM over the entire 60 minute analysis?

Second - I have to create my own historic traffic data tables.  In the help files it notes that you have the option of configuring historical profiles based on a percentage of travel speed versus free flow speed and values will be less than 0. Then it also briefly explains that if you are modelling travel times directly, values can be greater than 1, though it doesn't elaborate.

As our edges have a travel time value measured in seconds, it looks like I don't have to convert everything to a proportion.  Just not sure what network analyst will be looking for in terms of a value - if free flow is 100 seconds for example, and 8am is 120 seconds, would that make the value for the profile at 8AM 20 seconds (120-100)?  Or will it be looking for 1.2 (120/100)?  If it assumes it is getting a proportion, my value of 1.2 would indicate the link is faster than free-flow.  Perhaps I should just make my life easier and go with proportions of free-flow.

PatrickStevens

The first - is a service area analysis in network analyst 'dynamic' when configured with historic traffic data?  By that I mean, if we have travel times measured at intervals of 15 minutes, and I compute a service area of 60 minutes, does that mean network analyst will look to the travel times at 8:00AM, 8:15, 8:30, etc. as the trip progresses over the 60 minutes?

Yes.  Historical traffic is dynamic as you specify.  If your service area leaves a facility at 8:00 am, if it arrives at an edge at 8:23 am, the travel times/speeds for 8:23 are used.

Second - I have to create my own historic traffic data tables.  In the help files it notes that you have the option of configuring historical profiles based on a percentage of travel speed versus free flow speed and values will be less than 0. Then it also briefly explains that if you are modelling travel times directly, values can be greater than 1, though it doesn't elaborate.

As our edges have a travel time value measured in seconds, it looks like I don't have to convert everything to a proportion.  Just not sure what network analyst will be looking for in terms of a value - if free flow is 100 seconds for example, and 8am is 120 seconds, would that make the value for the profile at 8AM 20 seconds (120-100)?  Or will it be looking for 1.2 (120/100)?  If it assumes it is getting a proportion, my value of 1.2 would indicate the link is faster than free-flow.  Perhaps I should just make my life easier and go with proportions of free-flow.

The value in the Profiles table is a scale factor, so it will be multiplied times your speed (or time, if you use that).  If you are using speeds, for example, and your base speed is 100 KPH, then a value of 0.9 will give that road a speed of 90 KPH for the duration of that time segment.   If you are using time, and your time is 2 minutes, then a value of 1.2 will give that road a time of 2.4 minutes.

So, whether you use time or speed, the value in the patterns table is always a multiplier.  If you want to slow down speed, use a value between 0 and 1.  If you want to slow down travel time, use a value greater than 1.

Does that help?