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Adjusting manual turning counts with seasonal factors?

Adjusting manual turning counts with seasonal factors?

Adjusting manual turning counts with seasonal factors?

Do most people take turning counts at face value, or are they adjusted to reflect seasonal variation?

Reviewing an environmental assessment form for a new development the other day, I noticed the intersection capacity analysis was based on December turning counts. The overall evening peak LOS will be E, with one approach v/c at 1.01, and another v/c will be 0.91. Buying right of way to widen the failing approach is not an option because it's between two historic cemeteries. It's a roundabout, so re-timing isn't an option, either.

Our state DOT specifies a seasonal factor of 0.88 for December in our area. Assuming the turning count volumes are also ~88% of the year-round average, the intersection is likely to fail for most of the year.

Is this an appropriate way of looking at turning count data? I'm not sure seasonal factors for AADT measurements could be applied to something as fine-grained as a PM peak turning analysis. Intuitively, though, it makes sense to me that if the ADT for that month is likely to be below the AADT, then the same could be said for turning counts.

RE: Adjusting manual turning counts with seasonal factors?

In the UK we have the concept of 'neutral months' (April, May, June, September and October) which is when, ideally, traffic counts would be undertaken such that they were representative of a notional 'average' day, week or month. It looks like you have a similar concept. I would suggest that December has so many factors with the potential to affect traffic flow; adverse weather, the holiday season, etc., that claiming it is representative of 'average' turning flows, even at a single junction, is...misleading...to be polite.

As to applying a seasonality factor to turning counts, I think that as an assessor you could do this as an exercise, to determine whether there may be the potential problems you seem to suspect, and as a reason that the consultants who carried out the assessment should re-examine it (with the aim to get counts carried out during a more representative month). But I don't think such a calculation should be used to simply refuse to approve the assessment out of hand, for the reason you suggest - flows at single junctions can be affected by many factors, the season being just one. (Although one would hope any such confounding factors, such as road works, would - as far as they were known - be spelled out in the assessment).

RE: Adjusting manual turning counts with seasonal factors?

In general I would consider a December count to represent something less than average conditions, but it depends on the location. For a site associated with commercial shopping areas a December count may represent a higher than average tally.

Failure at opening is a cause for concern and the conditions that generate the approach failure are important. Is it due to high circulating volume, high approach volume, a combination of both, unbalanced volumes? What is the duration of the failure? A 10 minute failure may be an acceptable level of operation. What model was used to evaluate the operation? Was the design geometry developed from the intersection counts or was a concept simply overlayed on the intersection? Is the designer an experienced roundabout designer? An experienced designer should be able to answer those questions.

When planning roundabouts we utilize the current traffic counts to set the basic geometry based on the initial analysis. Then we project future traffic onto the design at about 5 year intervals to evaluate the sensitivity of the approaches to future traffic growth and get an estimate of the projected service life. Does the design need to accommodate future expansion? When? Is expansion possible?

RE: Adjusting manual turning counts with seasonal factors?

teeman, this is an evaluation of traffic to be added to an existing roundabout by proposed university dorm.

Since we can't modify the one approach, I suppose we could try to slow down the conflicting movements. The main conflicting movement is the university's driveway. The left turn from that approach is the one with the 0.9 v/c. Also, we'd lose the ensuing political argument if we suggested it.

I recommended that the university should implement traffic demand management measures such as transit incentives, increased parking fees, and staggered work hours. I'm not sure what other suggestions to make.

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