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Thread: ITE trip generation and parking rates

  1. #1
    Cyburbian transguy's avatar
    Jan 2006

    ITE trip generation and parking rates

    An article crossed my desk yesterday by Donald Shoup that talked about the major flaws of using average trip generation and parking rates by public agencies in determining the minimum requirements for development. While I agree with the major criticisms of the article, no clear and reasonable alternative method is presented to address the shortcomings of these rates (few data sources, primarily suburban locations with free parking ...).

    I think that most everyone would agree that having a great source of local data to assist in the creation of requirements for new/modified development; however, the costs generally associated with such a task prevent most of us from having this data.

    Does anybody know of any agency that routinely uses sources other than the "national averages" put out by ITE, ULI and others? For instance, do you have provisions that were the public or private agency has sufficient data to contradict the national average, you would allow the requirements to be based on the "alternative" (or primary) data? Any other ideas of how agencies are trying to deal with this shortcoming?

  2. #2
    Cyburbian Cardinal's avatar
    Aug 2001
    The Cheese State
    I am no transportation planner, but I recognize the severe limitations of ITE data. If there is a legitimate argument to use other resources, I would be inclined to accept it. For instance, on one project involving ethanol production, I used a combination of plant demand for grain, and results from a multi-state study of grain elevator traffic to construct an opinion of potential traffic generation.
    Anyone want to adopt a dog?

  3. #3
    Cyburbian Random Traffic Guy's avatar
    Jul 2005
    Lone Star State
    I swear, when I win the lottery I will set up an endowment at A&M to study this stuff that doesn't get enough attention. Mixed-use internal capture would be the main theme, followed by two-way stop controlled intersection delay.

    Anyways, local data is always good but takes money and time. We share data across our national company as well. I find most of the locals are very open to this, any info helps them make decisions and they can use the data in other cases as well. The bad things happen when the agency doesn't accept local data since it contradicts their preconceived notions. There's going to be a kids sports complex here with a sea of unused parking since the city didn't think it could share parking with the office building next door. Yes, the comparative local isolated site's parking was 100% full at peak times. But it doesn't have 1,000 extra spaces adjacent which will be empty nights and weekends.

    The other avenue, which sometimes has surprising amounts of data if you just ask, is to look at market projections and logically determine trip rates or parking accumulation from that. This works best in industrial applications when the user knows exactly how much material the site will be able to move or produce, and the traffic to support that can be determined. Also good for sports facilities with known user populations.

    All this said, I don't have a problem using ITE or ULI numbers with a sanity check. It is the available research. When designing sites I will listen to alternate proposals if there's any data backing them.

    Part of the problem is also perception and what are the goals of the parking. Can we provide parking for normal activity and have a few days of the year with problems. Or provide for the peak days and have a lot of unused concrete most of the year. How sensitive is the city and the owner to complaints (grow a spine!). Most of the parking oversupply I see comes from retailers expactations and perceived need for 5 per 1000sf, not their actual uses. As sites get tighter and more redevelopment happens, their expectations will come into line. You can wheather alot of complaints with $15k per space structured or $30k per space underground sitting in the bank rather than concrete on the ground.

  4. #4
    maudit anglais
    May 1997
    I think we've discussed this issue before. RTG made some very good points with which I agree. There is definitely a huge gap in available trip generation data. Big Box Power Centres/Lifestyle Centres is one missing land use which I'd like to see in the next version of the manual.

    Typically, I find it is the smaller jurisdictions that rely more on ITE as the "bible" for trip generation. This is logical given that a) they usually do not have the data or resources to develop localized travel data, and b) travel patterns in those areas tend to be reflective of the ITE data sources (e.g. little non-auto traffic).

    The jurisdiction I do most of my work in is fairly large (+/- 1 million people) and they have a decent (but not great) travel survey program (origin-destination survey every 5 years, cordon counts (every other year), intersection counts). At one point they had developed their own trip generation manual, but this is no longer used as the rates are not found to be accurate anymore. This is another issue with the ITE manual - the trip data is often very old, but you have no way of knowing that.

    What I typically do when undertaken trip generation is adapt ITE rates to local conditions by factoring in average auto occupancy and a value (typically 5%) to capture non-vehicle trips which the ITE survey doesn't pick up. This gives a decent approximation of "person-trips". I can then use local travel data to determine likely mode splits and get trip generation for all modes (required here). Factor in the auto occupancy factor again and you get your vehicle trips.

    Once in a while we get enough budget that we can do a proxy count for a similar use, in a similar location...this is the approach most jurisdictions find preferable.

  5. #5
    Aug 2008
    Yuba City, CA

    ITE rates

    There are some research studies by the California DOT and TRB that will affect changes to the ITE trip generation rates, especially for parking requirements at TODs. This has been a big problem in making TOD pencil out. The new rates should allow better formulas to allow TODs to win approval.

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