Clearance Times

There is an interesting article that appeared in the January 2008 ITE Journal written by Steven M. Click Ph.D., P.E. Application of the ITE Change and Clearance Interval Formulas in North Carolina. It provides a good analysis of clearance time interval determination. You need to log into the ITE site to read the article or find a copy of the journal.

Among other things, it shows the importance of timing intervals specific to the particular intersection not a one size fits all determination, especially for the red clearance or all red interval. Unfortunately, it doesn’t address bicycle timing, which can be a concern, if your controller provides the capability of separate bicycle clearance intervals like the Naztec controller can.For easier field review, I would suggest the preparation of a table with intersection clearance distance on the X-axis and approach speeds on the y-axis. Then provide the calculated vehicle yellow clearance, vehicle red clearance, and bicycle red clearance intervals for each combination of speed and distance. The table should show the actual controller settings, not the clearance intervals so red clearance is actually your minimum red clearance setting unless yellow clearance subtracted from the calculated red clearance is greater than your minimum red clearance. It is easily prepared using a spreadsheet. The formulas could be tailored to your agencies specific methods. The table would allow quick review in the field, without the need to run through the calculations each time. The more detailed timing determination can be performed at the design stage but so many things can slip by, like a new crosswalk, a new lane, or some other change that could affect timing. The “tool” would allow the check to be part of a regular review process.

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