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Safety effects of part-time signals?

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ACtrafficengr

ACtrafficengr

Civil/Environmental
Jan 5, 2002
1,641
On one of our arterials (3 lane, 30 mph, ADT = ~15,000) the detection has failed on a signal serving an office park. We've been getting complants about unnecessary delays due to the sideroad being on recall.

Little traffic comes out of the sideroad after about 6 PM. Several people have suggested putting the signal on flash at night. Has any research been done concerning whether the increased risk of right-angle crashes outweighs the reduction in delay?

This road is due for reconstruction in 2009, so we are loath to make the permittee spend a lot to fix the detection on a signal that may be rebuilt or even replaced by a roundabout in 2 years.

"...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

 
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I would say that the risk of right-angle collisions is more traffic and intersection geometry-related than flash/stop-and-go signal related. (In other words that research would suffer the causation-vs-correlation dilemma.) If the intersection does not meet a warrant criterion in the off-peak and does not have a history of accidents, especially if it is well-lit, then you'd be fine to put it onto flash mode in the off-peak.
 
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I have found a few references since I posted the question:

NCHRP REPORT 457: Engineering Study Guide for Evaluating Intersection Improvements, p. 13:
Benioff et al. (13) define conditions in which flash mode is not likely to cause safety problems. Specifically, they recommend using flashing yellow/red when (1) the total major-road volume is less than 200 vehicles per hour (veh/h), or (2) when the ratio of major-road-to-minor-road volume is greater than 3.0.
Click to expand...

and Abdelghany and Connor, (Guidelines for Operating Traffic Signals during Low-Volume Conditions, p. 30: state:

Based on the findings of the different studies presented above, it could be easily concluded that the safety of flashing traffic signals, especially yellow/red flashing mode is questionable. Most of the research showed a statistically significant increase in accidents and especially right-angle accidents, when implementing yellow/red flashing signal. In addition, most of the research tried to identify the relation between different
intersectional characteristics and accident rate when using flashing traffic signals.
Click to expand...

They go on to recommend that, after considering sight distance, speed, crash history, etc, red/yellow flash may be used during hours when the volumes would work with TWSC, and red/red flash could be used when AWSC would work.

"...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
 
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