When preparing traffic forecasts for a new development with multiple buildings on the site, traffic engineers typically reduce the trip generation (amount of traffic going in and out of the site) to account for patrons who go to the site and stop at more than one of the buildings. For instance some of the folks going into a Walmart will also stop at the McDonald’s located on the corner of the site.
Here are the four methods I’ve encountered for accounting for this deduction:
- ITE’s Trip Generation Handbook, 2nd Edition: There’s a specific methodology laid out and the underlying data is based on a couple of mixed use sites in Florida. If your site is all commercial/retail, the tables suggest using a 20% reduction in trips during the p.m. peak hour.
- NCHRP Report 684 – Enhancing Internal Trip Capture Estimation for Mixed Use Developments: Similar methodology to the ITE version above, but expanded the dataset from three sites in Florida by adding in three sites in Texas. Looks like the average trip reduction is about 13%.
- US EPA Mixed-Use Development Trip Generation: Based on a much larger sample size and requires much more detailed inputs. I used it once and my internal capture results were 8 % to 11% reduction factors depending on the time-frame.
- Use a 10% across the board reduction: I typically use this blanket reduction as do most of the traffic engineers in Minnesota. Given the above datasets, I think it’s reasonable if not slightly conservative. It’s also quick to use.
Gathering data on internal trips is very tedious. Call out to AirSage – I wonder if we can look at cell phone usage within mixed use developments to tease out how many stops people make within the site. Our industry could use a lot more robust dataset/methodology.