Conducting a Trip Generation Study

When to Collect Data Not covered by the ITE land use classification Location is in CBD Significant multi-modal component Size of site is not within ITE data range Insufficient sample size Inadequate statistical confidence Inconsistent from sites included in existing database (age of res., worker shifts, diff. ind. Var.) Multi-use development Questioned by professionals or local officials

Sample Size Determination Establish local trip generation rate  at least 3 sites preferably 5 Validate ITE trip generation rate  at least 3 sites Combine local data with ITE data  at least 2 sites Submit data to ITE  at least on site

Site Selection Transferrable (dev. Size, development mix, location w/r to network and dev. Patterns) At least 85% occupancy At least 2 yrs old Independent variable data available Can isolate site for counting (no shared parking, no shared driveways, limited ped traffic from nearby, limited transit, no through traffic) Security Single land use activity No construction Owner permission

Independent Variable Selection Available Variable size would affect trip generation Should be accurate not estimated or derived Currently used in manual

Development Data Requirements Site description –Square footage/units –Percent occupancy –Site acreage –Location (CBD, suburban, rural) –Name and description of principal use Site plan Adjacent street traffic volumes

Survey Periods Automatic count –7-day period ideal –1-day period at a minimum –2-day preferable Manual counts –2-hours for each peak Season –No-variation then pick average day –Variation then pick 30th or 50th highest hour Time of survey should represent typical activity (good weather, no big sale, etc.)

Conducting the Study Directional volumes 15-minute periods Two or more days of peak period counts Adjacent street hourly traffic counts for establishing peak period time Concurrent counts Site data from owners Verify automatic counts with short-period manual counts If needed, vehicle classification and vehicle occupancy

Establishing a Local Trip Generation Rate of Equation Hypothesis for why local conditions are unique Confirm that local data justify local trip generation rate/equation 3 sites (5 preferable) Difference of average greater than +/- 15% Local data consistent Rate or equation satisfies statistical standards For ave: –At least 3 data points –Std/ave <= 1.10 For Equation: –At least 4 data points –R2 >=0.75

Validation of Trip Generation Rates/Equations for Local Use STEP 1: Collect data at 3 sites or more STEP 2: Analysis of the local data, comparing to ITE –ITE data valid if… Criteria 1: trip generation rate for local site within +/- std Criteria 2: at least one site rate higher and one lower OR all with 15% Criteria 3: local data generally fall within ITE scatter Criteria 4: common sense If all of the criteria of STEP 2 are not met then consider local rate or equation

Combining Trip Generation and Local Data Combine if ITE and local rates within 15% Does not give precise std or regression equation Sum trip ends(ITE) = weighted average * average X * number of studies Sum independent variable units (ITE) = average X * number of studies