RE: [CTPP] CTPP Part 3

Jiji: Ken is totally correct. You need to convert the CTPP commuter matrix into something like an "observed home-based work" trip table using trip rates per employed residents from your local travel survey, the NCHRP #365 (1.50 trips/worker), or the NHTS. Remember that the census only provides a count of workers, and is not a count of work trips, per se. We (SF Bay Area) did *not* use the CTPP2000 Part #3 commuter matrices in our Year 2000 model validation project. Our model validation project ended in April 2004, and the CTPP Part 3 data was available May 2004. Such is life. On the other hand, we did use the 5% PUMS data (available August 2003) to construct what we needed: county-to-county commuters by means of transportation by household income quartile. We then converted these commuter matrices into "observed home-based work" trips. For our validation, we sequentially validated our auto ownership (to Census PUMS data), work trip generation, work trip distribution, and work trip mode choice models to these census-based observed databases. We then went through some fairly exhaustive "network validation" stages where we compared the model simulated traffic (daily and peak) and transit (daily) to observed traffic and transit counts. At this network assignment validation stage we diverged from our "observed HBW" trips in order to better match observed traffic/transit databases. The basic problem we were having with the PUMS-based observed work trips was that it was too low (transit, especially) in our Transbay corridor (SF to/from the East Bay). This is a problem with either too few commuters in the decennial census, or perhaps our trip rate for this distance of work trip was too low. So, my recommendation is to use the commuter matrices from the CTPP, convert them into appropriate "observed home-based work" trips, then do initial stages of validation against these observed databases, BUT be prepared to diverge from these CTPP-based observed trip tables if and when your transit/traffic assignment validation is inadequate. OR, you can always "hold true" to the CTPP-based observed data, and focus your attention on calibration / hammering away at your non-work trip models. Cheers, Chuck Purvis, MTC-Oakland 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(a)chrispy.net [mailto:ctpp-news-bounces@chrispy.net]On Behalf Of Jiji Kottommannil Sent: Monday, August 16, 2004 12:09 PM To: ctpp-news(a)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