On your question #1: One thing to keep in mind relates to the fact that a "HBW trip" for modeling purposes represents a trip end, e.g., a Home to Work trip and a Work to Home trip would actually be counted as two separate trips, i.e., HBW productions and two HBW attractions.
So, using your NCHRP example, 1,000 employees would generate 1,500 HBW trips--but only half of these (750) would be comparable to a CTPP-derived home-to-work trip.
In terms of why travel survey data never supports 2.0 HBW trips per employee: It is partly because of absenteeism on an "average" weekday, partly because some jobs are simply not five-day-a-week jobs, and partly because some (or even many) people don't actually go straight from home to work (or from work to home) on their surveyed day (e.g., they engage in some trip chaining, which turns some Home-to-Work trips into HNW and NHB trips, and turns some Work-to-Home trips into NHB and HNW trips).
Hope this helps!
sincerely,
Ken Cervenka
NCTCOG
-----Original Message-----
From: ctpp-news-bounces@chrispy.net
[mailto:ctpp-news-bounces@chrispy.net]On Behalf Of Jiji Kottommannil
Sent: Monday, August 16, 2004 12:09 PM
To: ctpp-news@chrispy.net
Subject: [CTPP] CTPP Part 3
Hi All,
I am a new user of the CTPP data and am in the process of comparing CTPP
part 3 tract-tract worker flow data to the Home-Based Work trip table
from the travel demand model I am currently working on. I came across a
few questions/issues for which I would welcome your
thoughts/experiences:
1) Do the CTPP part 3 flows account for all the home-based work trips? I
noticed that the commonly accepted home-based work attraction rate is
about 1.5 trips per total employment (NCHRP 365) and this creates more
HBW trips than there are in the CTPP part 3. Let me know if I there is
something wrong in my interpretation of the data.
2) How much success have the MPO's had in calibrating their trip
distribution models based on CTPP Part 3? What level of accuracy is
typically desired?
Thanks!
Jiji
Jiji V. Kottommannil
Transportation Modeling Specialist,E.I.T.
Crawford Bunte Brammeier
Phone: 314-878-6644, Ext. 38
Fax: 314-878-5876
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