Here are a few reasons why the sum of the O-D Index values might be less than the Zone Traffic Index values:
Somehow trips are being double counted. This happens two ways:
- Zones are overlapping. If you’re working with Pass-Through Zones and some trips travel through both Zones, then you may be capturing the same trip multiple times. For example, if a trip originates in Zone A, passes onto Ramp B and then later onto Road Segment C, both B and C could be counted as Destinations and the trip could be counted twice.
- Errors due to Rounding. The way trips are "binned" into different time periods may also cause a difference in values. If trip leaves an Origin at 2pm and arrives at a Destination past 3pm, we would attribute this to the Origin hour and not to the Destination hour, potentially causing some discrepancies between how trips are broken down between the O-D and Zone traffic indexes. This is more likely to impact Pass-Through Zones.
If the sum of the O-D Indexes is significantly more than the Zone Traffic Indexes, then it’s likely that there is double counting.
If the sum of the O-D Indexes is only slightly greater than the Zone Traffic Indexes, then it’s more likely that the difference is related to rounding errors or trip binning by time period.