JARGON ALERT – I am trying to clarify a concept here that is wrapped in traffic engineering jargon.
I have had some difficulty explaining pass-by trips to a client. I previously discussed how much traffic is generated by proposed developments in my post on trip generation. Please review that if you need to get up to speed on the topic.
Pass-by trips are a subset of trip generation that only apply to commercial/retail developments. They are the folks already on the road who the business hopes to suck into their site as they are driving by. Think about a gas station.
A "new trip" for a gas station is me running out of gas for the snow blower, jumping into the truck, going to fill up my gas can, and then going back home to use the snow blower. A "pass-by" trip is when I am driving to a meeting, I realize I am out of gas, and I pull into the next gas station driveway I pass. I was already on that road and I am going to continue my trip to the meeting after I fill up. The third type of trip is the "diverted" trip. I am driving to a meeting, I realize I am out of gas, I have a coupon for a gas station that is three blocks out of the way, I get off of my main path to go to the gas station, and then I go back to my main path after I fill up.
New trips and pass-by trips are straightforward. You are either going to the business and returning the way you came OR you happened to by driving by, you stop in because it is convenient, and then continue on your merry way.
The trouble comes in with diverted trips because they depend on how far out your study goes. Think about stopping in at Home Depot on the way home from work to grab a box of nails. You have to get off of the freeway and drive half a mile on a county road to get there. Then you'll get back on the freeway to go home. If we are only analyzing the intersection on the corner of the Home Depot, this trip is going to look like a new trip because you are coming and going in the same manner through the study intersection. Now if you are also analyzing the two ramp intersections at the freeway interchange, you have to treat it as a diverted trip. You are coming and going from the same direction at the intersection on the corner of the Home Depot, BUT you are continuing onto the freeway in the same direction you started from. You are not returning to the place where you originally got into your car.
Thankfully, diverted trips only come into play in traffic studies for large, regional shopping destinations.