Traffic Signals for Dummies

The article below made me think that a post explaining how engineers decide where to put traffic signals could be useful.

Residents Push For Stop Lights At 6 Intersections
Thursday, Jul 31, 2008 – 04:01 PM Updated: 06:10 PM
By Denise Yost

COLUMBUS, Ohio — Central Ohio communities have noticed an increase in the frequency of crashes at local intersections.

The group said there are six intersections in Central Ohio that they call extremely dangerous. They are the scenes of many car crashes, and the group said the big problem is that there are no stop lights at the locations, NBC 4’s Matt Alvarez reported.

Residents have complained to the Columbus Transportation Commission about the intersections, which include Mock Road at Woodland Avenue, Americana Parkway at Tussing Road, Brentnell Avenue at Mock Road, Broad Street at Cedar Cliff Drive, Cidermill Drive at Wilson Road and Grimes Drive at Wilson Road.

The Columbus Transportation and Pedestrian Commission’s job is to examine residents’ concerns on city streets, sidewalks and other transportation issues they encounter.

In a May meeting, members looked at the intersections, and what motorists want aren’t stop signs, but stop lights.

After studying the locations, the commission has approved only one stop light — for the intersection of Broad Street and Cedar Cliff Drive. The other locations were denied for not meeting proper requirements, like not enough traffic volume.

So this is basically what I do for a living at the moment. In Chicago, it goes like this:

1. Citizens request stop signs.
2. Aldermen forward the requests to the Department of Transportation.
3. We (traffic engineers) tell the Aldermen that stop signs on major streets are an extremely bad idea but offer to look into other options, like traffic signals.
4. We do a study to see if signals are “warranted.” They rarely are, so we try to suggest other solutions to the real or perceived problems at the intersection.

So how do we decide if a traffic signal is “warranted,” meaning that signals should be installed at an intersection? Basically we follow standards in the Manual on Uniform Traffic Control Devices (MUTCD). The MUTCD, published by the Federal Highway Administration (FHWA), establishes national standards for traffic control devices (signs, markings, signals, etc…). It has been adopted as law in some form by all 50 states and the District of Columbia. The Manual is revised every several years, primarily based on transportation safety research. A new edition is currently in the comment phase and should be released in about a year.

So what are the standards?

1. Volume

By volume, I mean the total number of vehicles (or pedestrians) on a street in a given time period. Minimum vehicle and/or pedestrian volume thresholds must be exceeded on the major and minor streets for a certain number of hours in an average day. The four-hour volume warrant requires a higher volume of traffic than the eight-hour volume warrant. Minimum volumes vary by number of lanes on the major and minor streets. Reduced standards can be used at locations with high speeds, in isolated small towns, or if other engineering options have failed to solve a safety problem. Here are some tables and graphs:

Warrant 4 – Pedestrian Volume

Must be 100 or greater for any four hours or 190 for any one hour and there must be fewer than 60 gaps in traffic per hour (or one gap per minute) in which pedestrians can cross the street in the same time periods when the volume criteria are met.

Just because a volume warrant is met doesn’t mean a signal will be installed though. If most of the turns are right turns or if there is little delay for vehicles or pedestrians, then there may be no need for a signal despite the high traffic volumes. The Manual allows for a lot of leeway with engineering judgment, but most engineers won’t vary from it too much. Sometimes engineers have to give depositions for law suits.  Failing to follow the Manual leaves open the possibility that you could find yourself in court trying to explain why you deviated from national design standards, and could jeopardize the status of your Professional Engineering license.

2. School Crossing

Warrant 5 is basically a modified pedestrian volume warrant. It allows a signal to be installed for a school crossing where there are at least 20 students crossing during an hour and fewer than one gap in traffic per minute when students are crossing.

3. Signal Coordination

Sometimes an additional signal in a large gap between two signals can help to keep vehicles grouped together in platoons, which improves the progression of traffic, regulates speed, and provides pedestrians with more gaps to cross the street.

4. Crash history

There need to be five or more reported crashes of type susceptible to correction by a traffic signal within a 12-month period and to quote the MUTCD, “adequate trial of alternatives with satisfactory observance and enforcement has failed to reduce the crash frequency.”

It is important to remember that traffic signals are not safety devices. They are traffic control devices meant to assign right-of-way. A signal installed at a location not meeting warrants could increase the number of crashes.

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