Trip distribution

[Surcharged]

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.

 

[TerryScan]

One approach I use is to analyze the existing traffic counts. I will look at a nearby side road traffic generator on the road in question and see where the distribution is going. It is likely that the proposed subdivision will follow the same pattern.

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.

 

[francesca]

If there is a nearby subdivision of similar size and house type, you can spend a morning and evening doing a turning count and extrapolate based on the number of lots. Assuming you get a good match and time your survey appropriately (Tues-Thurs, while school is in session) this is actually more accurate than the ITE's trip generation rates, with a bonus that you get turning movements too.

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.

 

[Surcharged]

Looking at an existing subdivision and side road is a great idea - I should have thought of that. We have a pneumatic tube traffic counter. I am assuming that both of you are talking about doing hand counts, right? Is it possible to arrange the counter so that I can determine turn movements on the side road and/or an existing subdivision entrance. I would like to have more than 1 hours worth of data.

 

[ACtrafficengr]

If the road has separate left and right turn lanes, you might be able to get decent turn counts with an inductance counter in each lane. The problem with tube counters near intersections is if someone stops on the tube, you won't get any counts until it moves, and turning traffic may give you 4 hits instead of 2.

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

 

[francesca]

You can count for an hour (AM and PM) and assume the same turning movements for the rest of the data, but be careful if there is a distinctly different employment character of each town as blue collar tends to start 1-2 hours earlier than white collar, not to mention retail. Someone may have to take one for the team and be there 6:30am - 8:30am and 3:30pm - 6:30pm (to get the school trips in the PM).