5 Aug
2009
5 Aug
'09
7:01 a.m.
The seminar I went to two years ago seemed to indicate minimum TAZ size in the range of
1200 people (again, pop or employment or combination). The issue is that in most areas
transit accounts for 0.5-1.5% of the mode split and if you want to keep from having that
data suppressed you will need to have that many people in it. I was told that suppression
usually will occur when the real value is 3 or less, but true integer value reporting
doesn't start until the real value is more than 10 (I think...it's been 2 years).
The problem then becomes that area totals over several TAZ's are way off because you
are adding "rounded" numbers rather than real numbers.
I suspect that the real question will have to do with which data set you are going to use.
If you are going to try to use the ACS 1 year aggregate you really need large TAZ's.
They say they are not going to suppress as much in the 3 year aggregate and may not
suppress anything in the 5 year (at least for category totals).
As a modeler, this drives me completely crazy. To get the network to load well, you need
to have the TAZ size and connections that are appropriate relative to the geography and
roadway network. If you're dealing with a subdivision with 500 houses and one or two
driveways the 1200 people limit is fine. If you're dealing with a city block with
single family units, you've got a real problem.
One way to get around this (thinking out loud) is to take an area with a relatively
uniform density and report that area as a whole as a census TAZ. Then when you get the
data back, you can split that TAZ proportionately for your model's sake. Most travel
demand models are only accurate enough to get you lane calls anyway so making some
internal estimates from the Census data will keep you away from suppression issues and not
adversely impact your accuracy. It will take a little data massaging on the back end but
it may be worth it, especially in areas with dense roadway network connections.
Patricia C. Tice, PE, AICP, LEED AP
Glatting Jackson Kercher Anglin, Inc.
120 N. Orange Ave
Orlando, FL 34787
407-284-4753
5 Aug
5 Aug
8:41 a.m.
New subject: ctpp-news Digest, Vol 66, Issue 1
Hello:
We have several TAZ's that have zero population and zero employment because we are
modeling areas that are entirely agricultural. One area is about 2 square miles. We also
have some TAZ's in central portion of the modeling areas (industrial) that are
distinct in terms of the surrounding land use (residential). For us, the issue has always
been getting TAZ's are similar in landuse and are defined by access to the
transportation network rather than population.
Last Census we split everal TAZ's that were developed before we did a revision to our
model and it was a real complex problem to split TAZ's in terms of creating matrixes
and balancing the network. I would advise against that procedure unless absolutely
necessary.
Is the Census Bureau going to enforce this rule of 1200 (employment + population) for
Taz's as part of the 2010 census?
Mike LeBlanc, AICP
Planning Manager, Special Projects Division
Rosa Parks Transportation Center
Lafayette Consolidated Government
Department of Traffic and Transportation
P.O. Box 4017-C
101 East Cypress Street
Lafayette, LA 70502
Ph: 337-291-7179
Fx: 337-291-8458
Planning for the 21st Century
-----Original Message-----
From: ctpp-news-bounces(a)chrispy.net [mailto:ctpp-news-bounces@chrispy.net] On Behalf Of
Patricia Tice
Sent: Wednesday, August 05, 2009 7:02 AM
To: ctpp-news(a)chrispy.net
Subject: [CTPP] RE: ctpp-news Digest, Vol 66, Issue 1
The seminar I went to two years ago seemed to indicate minimum TAZ size in the range of
1200 people (again, pop or employment or combination). The issue is that in most areas
transit accounts for 0.5-1.5% of the mode split and if you want to keep from having that
data suppressed you will need to have that many people in it. I was told that suppression
usually will occur when the real value is 3 or less, but true integer value reporting
doesn't start until the real value is more than 10 (I think...it's been 2 years).
The problem then becomes that area totals over several TAZ's are way off because you
are adding "rounded" numbers rather than real numbers.
I suspect that the real question will have to do with which data set you are going to use.
If you are going to try to use the ACS 1 year aggregate you really need large TAZ's.
They say they are not going to suppress as much in the 3 year aggregate and may not
suppress anything in the 5 year (at least for category totals).
As a modeler, this drives me completely crazy. To get the network to load well, you need
to have the TAZ size and connections that are appropriate relative to the geography and
roadway network. If you're dealing with a subdivision with 500 houses and one or two
driveways the 1200 people limit is fine. If you're dealing with a city block with
single family units, you've got a real problem.
One way to get around this (thinking out loud) is to take an area with a relatively
uniform density and report that area as a whole as a census TAZ. Then when you get the
data back, you can split that TAZ proportionately for your model's sake. Most travel
demand models are only accurate enough to get you lane calls anyway so making some
internal estimates from the Census data will keep you away from suppression issues and not
adversely impact your accuracy. It will take a little data massaging on the back end but
it may be worth it, especially in areas with dense roadway network connections.
Patricia C. Tice, PE, AICP, LEED AP
Glatting Jackson Kercher Anglin, Inc.
120 N. Orange Ave
Orlando, FL 34787
407-284-4753
_______________________________________________
ctpp-news mailing list
ctpp-news(a)chrispy.net
http://www.chrispy.net/mailman/listinfo/ctpp-news
6 Aug
6 Aug
11:10 a.m.
New subject: ctpp-news Digest, Vol 66, Issue 1
As I recall my old modeling lessons, sometimes you need to create "dummy TAZs"
Whether you would get Census data for it or not that is a different issue.
Ali Bonakdar, AICP
Director
Corvallis Area Metropolitan Planning Organization
301 SW. 4th Street, #140
Corvallis, OR 97333
Ph: (541) 758-1911
Fax: (541) 758-3127
Email: abonakda(a)ocwcog.org
Web: www.corvallisareampo.org
-----Original Message-----
From: ctpp-news-bounces(a)chrispy.net [mailto:ctpp-news-bounces@chrispy.net] On Behalf Of
Mike LeBlanc
Sent: Wednesday, August 05, 2009 6:42 AM
To: 'ctpp-news(a)chrispy.net'
Subject: [CTPP] RE: ctpp-news Digest, Vol 66, Issue 1
Hello:
We have several TAZ's that have zero population and zero employment because we are
modeling areas that are entirely agricultural. One area is about 2 square miles. We also
have some TAZ's in central portion of the modeling areas (industrial) that are
distinct in terms of the surrounding land use (residential). For us, the issue has always
been getting TAZ's are similar in landuse and are defined by access to the
transportation network rather than population.
Last Census we split everal TAZ's that were developed before we did a revision to our
model and it was a real complex problem to split TAZ's in terms of creating matrixes
and balancing the network. I would advise against that procedure unless absolutely
necessary.
Is the Census Bureau going to enforce this rule of 1200 (employment + population) for
Taz's as part of the 2010 census?
Mike LeBlanc, AICP
Planning Manager, Special Projects Division
Rosa Parks Transportation Center
Lafayette Consolidated Government
Department of Traffic and Transportation
P.O. Box 4017-C
101 East Cypress Street
Lafayette, LA 70502
Ph: 337-291-7179
Fx: 337-291-8458
Planning for the 21st Century
-----Original Message-----
From: ctpp-news-bounces(a)chrispy.net [mailto:ctpp-news-bounces@chrispy.net] On Behalf Of
Patricia Tice
Sent: Wednesday, August 05, 2009 7:02 AM
To: ctpp-news(a)chrispy.net
Subject: [CTPP] RE: ctpp-news Digest, Vol 66, Issue 1
The seminar I went to two years ago seemed to indicate minimum TAZ size in the range of
1200 people (again, pop or employment or combination). The issue is that in most areas
transit accounts for 0.5-1.5% of the mode split and if you want to keep from having that
data suppressed you will need to have that many people in it. I was told that suppression
usually will occur when the real value is 3 or less, but true integer value reporting
doesn't start until the real value is more than 10 (I think...it's been 2 years).
The problem then becomes that area totals over several TAZ's are way off because you
are adding "rounded" numbers rather than real numbers.
I suspect that the real question will have to do with which data set you are going to use.
If you are going to try to use the ACS 1 year aggregate you really need large TAZ's.
They say they are not going to suppress as much in the 3 year aggregate and may not
suppress anything in the 5 year (at least for category totals).
As a modeler, this drives me completely crazy. To get the network to load well, you need
to have the TAZ size and connections that are appropriate relative to the geography and
roadway network. If you're dealing with a subdivision with 500 houses and one or two
driveways the 1200 people limit is fine. If you're dealing with a city block with
single family units, you've got a real problem.
One way to get around this (thinking out loud) is to take an area with a relatively
uniform density and report that area as a whole as a census TAZ. Then when you get the
data back, you can split that TAZ proportionately for your model's sake. Most travel
demand models are only accurate enough to get you lane calls anyway so making some
internal estimates from the Census data will keep you away from suppression issues and not
adversely impact your accuracy. It will take a little data massaging on the back end but
it may be worth it, especially in areas with dense roadway network connections.
Patricia C. Tice, PE, AICP, LEED AP
Glatting Jackson Kercher Anglin, Inc.
120 N. Orange Ave
Orlando, FL 34787
407-284-4753
_______________________________________________
ctpp-news mailing list
ctpp-news(a)chrispy.net
http://www.chrispy.net/mailman/listinfo/ctpp-news
_______________________________________________
ctpp-news mailing list
ctpp-news(a)chrispy.net
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2 comments
3 participants
participants (3)
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Ali Bonakdar -
Mike LeBlanc -
Patricia Tice