Is It Time To Dump The ITE Trip Generation & Parking Manuals?

Or at least downgrade them from their status as gospel?

The ITE (Institute For Transportation Engineers) Trip Generation Manual collects data throughout the United States and Canada at low-density, single-use, homogeneous, suburban developments with little or no public transit service and little or no pedestrian access, and uses that data to create trip generation rates for different categories of uses.

ITE Parking and Trip Generation manuals have become the gold standard for planners and transport professionals in the U.S. and even abroad, even though the projections are often unmerited, statistically misleading, and rarely, if ever, verified once a project is completed.

The gospel, according to the ITE, is that building square footage has a reliable statistical relationship to both trip (auto and truck) generation for a particular use and off-street parking requirements. In fact, the gospel is repeated so often, that many, if not most, zoning and building codes base their minimum off-street parking requirements on the ITE manuals. Many local planning boards defer completely to ITE trip generation data in approving development projects over local resident opposition.

The folly in this logic (and the reliance on ITE estimates) is that the continuous acquisition of data for new developments as they come on line would necessitate that the quantities (parking or traffic) per square foot would be constantly evolving with new data, while zoning codes are infrequently, if ever, updated. And, there is also no meaningful road-testing and data collection related to the ITE generation dictates, post-project. That is, how many, if any, municipalities test their parking or traffic generation conclusions in the real world for accuracy, once a project is operating.

The precision with which the ITE tables report trip generation and parking needs indicates confidence in its accuracy, usually down to two decimal points, e.g.- a high cube fulfillment center warehouse generates 1.37 trips per 1000 square feet, while a specialty trade contractor generates 1.97 trips per 1000 square feet. It’s hard to argue with that sort of accuracy, but what is it really based on?

Municipal planners rely on parking generation rates to establish off-street parking requirements, and traffic engineers rely on trip generation rates to predict traffic effects of proposed developments. Yet a close look at the data shows that an unwarranted trust in these precise but uncertain estimates of travel behavior can lead to bad transportation, parking, and arbitrary land-use policies.

A survey of available literature suggests that of the more than 1,500 trip generation rates, about half are based on five or fewer studies, and about a quarter were based on a single study. ITE’s stamp of authority relieves planners from the obligation to think for themselves—the answers are right there in black and white. ITE offers a precise number without raising difficult public policy questions, although it does warn, “Users of this report should exercise extreme caution when utilizing data that is based on a small number of studies.” Some reported traffic data are also based on single or limited-day data collection.

In those instances when the ITE actually reports an R-squared value (a statistical measure between 0 and 1 that indicates a measure of variance in the data), planners should only utilize estimations with a sufficient number of data entries, and an R-squared factor of at .50 or higher. Given the deference given to these estimations, planners should have the highest level of certainty that the data is reliable.

Parking needs are often determined by blind adherence to national standards developed by industry groups' compilation of data without regard to the specifications of any particular proposed project. A bank is a bank is a bank, regardless of location. A warehouse is a warehouse is a warehouse, regardless of location. Same for multi-family housing options. This data becomes enshrined in local zoning codes for off-street parking requirements as though it were gospel. Large warehouse projects get approved near residential neighborhoods based on untested traffic generation estimates.

And. what about the changing nature of certain industries? Take just one example, retail banking. The shift to internet banking, the downsizing of staff, and the growing use of credit cards (in lieu of cash) to make payments for everyday items has certainly shifted the need for parking spaces at retail bank locations. Despite these changes, zoning codes continue to require minimum parking spaces that have no correlation to changes in the retail banking landscape.

And, it’s not just minimum off-street parking requirements. Trip generation tables that rely on outdated data are sure to overestimate the traffic generated by retail bank locations. This is just one example – it wouldn’t take much effort to list others.

Planners and officials argue that the ITE is the best data available. What they are really saying is that it’s the only data available, but reliance on a flawed system doesn’t validate its use. There are an adequate number of regional projects from which actual data can be gleaned to create an alternative system to at least check, if not replace the data offered by the ITE.