Trip distribution
Trip distribution
(OP)
I have a proposed subdivison that I am doing a traffic study. It is located on a state highway and in between 2 cities but closer to one city. The county has asked us to let them know if we recommend a left turn lane or a right turn lane. I want to determine the number of turns into the subdivsion. I can determine how many trips total based on the number of lots. Then I could assume that half of those are trips in and half of those are trips out. But How do I determine how many of the trips into the subdivision are going to be left turn ins and how many will be right turn ins? thanks in advance.





RE: Trip distribution
Another approach is to simply use one's judgement. This is where it is very helpful if one is familiar with the area. I have seen ridiculous assumptions made when a company is not familiar with an area. For a residential subdivision: where are the schools that will serve the property? where is the grocery store? (these are likely more consistant trip generators vs. the question of which of the two towns one may work in which may be more evenly split). You can come up with your own questions based on your familiarity with the area and potential trips and how often one makes each type of trip.
RE: Trip distribution
You would also need to look at the target demographic for the houses and the market dynamics of the towns. Is one rapidly expanding providing jobs at the right income level? Is one more blue collar/white collar than the other?
Never forget the old Gravity Model chestnut; trips will decline with distance squared. It's less of a factor if the distances are say 5 miles verses 10 miles, as a 10 mile commute is short in an uncongested network, but if the distances are 20 miles verses 40 miles, then you want to take it into consideration.
Lastly, a turning lanes are never a bad idea from a safety perspective, and if the county is putting them in for you, just say yes.
RE: Trip distribution
RE: Trip distribution
I am strongly in favor of left turn lanes since their effect on safety and capacity is so much higher. Left turn traffic is more likely to impede through traffic than right turn traffic, resulting in rear-end crashes, queues, and left turn/opposing traffic crashes due to drivers feeling rushed and choosing an inadequate gap.
For a stop controlled T intersection, the CMF for a left turn lane is 0.67 (urban) to 0.56 (rural) or a 33% to 44% reduction in crashes.
A right turn lane has CMFs of 0.74 (both approaches of a + intersection) to 0.86 (one approach), a 14% to 26% reduction.
These numbers are from Safety Effectiveness of Intersection Left- and Right-Turn Lanes, D.W. Harwood, et. al., July 2002, 1. Report No. FHWA-RD-02-089.
"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928
"I'm searching for the questions, so my answers will make sense." - Stephen Brust
RE: Trip distribution