How to Forecast Hourly Volumes for Signal Warrant Analyses...

Tim Aziere, PE, PTOE from Baughman Company in Kansas emailed me this interesting question –

I am doing a signal warrant for a proposed mix-use development here in Wichita. The existing land use is a golf course and the proposed uses (10 of them) will generate over 43,000 veh/day. I have little doubt that a signal at the major entrance (two entrances proposed) will be warranted, but I am having trouble finding a credible way of distributing the daily traffic for the warrant analysis. Is there a program I am not aware of, or an industry standard I could use?

Signal Warrants Background

There's a technical process for analyzing traffic volumes at an intersection versus different criteria in the Manual on Uniform Traffic Control Devices to determine if a traffic signal is needed at an intersection (the criteria are called warrants to confuse the layman). "Engineering judgment" is sprinkled in to let us professional engineers interpret the tea leaves. 

Most of the criteria are based on the busiest eight hours of traffic volumes.  Traffic impact studies typically just forecast one or two peak hours.  The big question is how to estimate the eight busiest hours of traffic based on the peak hour forecasts.

My Response on How to Forecast Hourly Volumes for Signal Warrant Analyses

I've been in your situation half a dozen times.  I don’t know of a perfect, industry standard method.  The two routes I’ve taken:

1. Get a 24 hour count on each of the roads your studying with the data broken down hourly (or a nearby road if things aren’t built yet).  Assume the same hour by hour distribution will apply before and after the development.  So if your peak hour is from 5-6 pm; your 4-5 pm hour is probably 95% of the peak hour, 3-4 pm hour is probably 85% of the peak hour, 2-3 am is probably 5% of the peak hour, etc.  I assume you have p.m. peak hour turning movement forecasts that you can add up to give you the total flows into the intersection.  Apply the hourly ratios to distribute based off of the p.m. peak hour having a factor of 1.0.     
2. Go to NCHRP Report 365 – Travel Estimation Techniques for Urban Planning.  It has a series of tables in Chapter 8 that give hourly distribution factors for different types of land uses sliced by the population size of the overall metropolitan area.  You can use those factors to factor your peak hour forecasts into hour by hour forecasts.

Neither method accounts for an area that has highly directional patterns (residents go to work in the a.m. and come back in the p.m.) and I wouldn’t use either method for an intersection that was right on the bubble of meeting warrants.  But these are reasonable methods to back up your engineering judgment if you strongly believe a signal is needed.

Comments

[Chet Skwarcan]

Mike - was not aware of the NCHRP Report, I'll have to check it out. What I have done in the past is draw upon previous studies where I collected driveway data for various residential/commercial developments. This historical data was invaluable when converting trip gen data to hourly data for this very purpose. And, if necessary, I would collect data from a similar development to get a feel for hourly distributions...