When using ONE HOUR (ODTAB) profiles, why does the total vehicles/hour in the final table of the ARCADY output file not match entered hourly counts?
Example: The hourly total count on an arm for, say, 08:00-09:00 is entered as 1000 PCU/hr, and a ONE HOUR (ODTAB) profile selected.
ARCADY will synthesise a peak demand profile that covers the central hour plus an extra 15-minute block either side of the central hour. The demand synthesised by ARCADY for each 15-minute time segment will therefore look something like this:
07:45-08:00 753 PCU/hr
The average over the central hour, i.e. 08:00-09:00, is (899+1101+1101+899) / 4 = 1000, which is the entered hourly count.
The average over the whole 90 minutes is slightly less: 918 PCU/hr. In other words, the average over the whole 90 minutes is slightly less than the entered hourly count because the synthesised profile includes a period either side of the central hour where the traffic demand is assumed to be lower than that measured during the central hour.
Using this profile type implies a number of assumptions, and of course the real demand profile at the junction may be completely different. For this reason, the ONE HOUR (ODTAB) profile should only be used if you know that the traffic profile at the junction follows a normal, typical peak period pattern.