Here are a few reasons why the sum of the O-D Index values might be less than the Zone Traffic Index values:
- The Origin and Destination Zones don’t represent a closed system. Therefore, there are trips passing through or starting in your Origin Zone that do not pass through or end in any of your Destination Zones and vice versa. This may be because trips start or end outside of your region of analysis or because trips take alternate routes.
- If you believe that your Zones represent a completely closed system, there is still the possibility that a few trips are escaping or leaking out between your Origins and Destinations. This might happen if GPS or LBS trips don’t perfectly lock to the road network.
- The way trips are "binned" into different time periods may also cause a difference in values. For example, if you're looking at AM Traffic between the hours of 7am and 9am and a trip passes through your Origin Zone at 8:59am and one of your Destination Zones at 9:03am, then that trip might be included in your O-D Traffic Index for that time period, but excluded from the Destination Zone Traffic Index.
If the sum of the O-D Indexes is significantly less than the Zone Traffic Indexes, then it’s likely that you have not created a completely closed system and trips are travelling along other roads or ending in locations outside your study area. When creating your Zone Set, you may want to turn on the OpenStreetMap ("OSM") layer in order to view all potential road segments in the area.
If you’re confident that your Zones represent a closed system and the sum of the O-D is only slightly less than the Total Zone Traffic, then it is likely the case that unlocked trips and binning are the cause for the discrepancy.