A Look at Traffic

Case studies to allow an analysis of traffic.

Wednesday, May 30, 2012

All About: Intersections, Part 2 of 2

This is a second part in a two part series to lock down some information about intersections, we go a bit more in depth with traffic signal operations at intersections. This information builds off of part 1 about some of the basic “surface level” knowledge of intersections. In part 2, we attempt to explain more advance signal operations and the future technology that has the potential to completely revolutionize signalized intersections and signal operations as we know it today.

Signal Phases

As mentioned in Part 1, most major intersections in major metropolitan areas, intersection signals are based on a pre-timed setting that allows for variation based on loop detector signals from vehicle arrivals. These pre-timed settings are determined by a traffic engineer (usually with the help of some software) and recorded on signal timing sheets for reference. A semi-actuated or fully actuated intersection would have the following (among a list of other values for signal operation), on signal timing sheets.

·         NEMA Phases
·         Minimum Green
·         Yellow
·         All Red
·         Walk time
·         Cycle length
·         Lag Phases

NEMA Phases – Each direction of traffic is assigned a numbered NEMA phase to organize a signals operation. Through directions are assigned even numbers in a clock wise direction usually with the major street labelled 2 or 6. Left turn movements are oddly numbered in a clock wise direction, with 1 being assigned across the intersection from the through 2 movement. Below is a diagram of a street layout with the NEMA phases labelled.

Minimum Green – The minimum allowed green time for a particular phase. This allows for a minimum clearance for just one vehicle to pass through an intersection. Usually 5-10 seconds for a medium sized intersection
Yellow – The amount of time given to the yellow phase of a light.
All Red – Each intersection has a period of time that has all the signal directions displaying red. After each phase, and a conflicting movement proceeds, the intersection has a “clearing” period to make sure all the yellow light runners have passed through the intersection completely.
Walk Time – The time allocated for pedestrians to cross the street when the pedestrian button is pressed/activated.
Cycle Length – Sum of all the phases green, yellow and all red times to complete one cycle of an intersection. There is usually a schedule for each day of the week and time of day for the cycle length to change accordingly.
Lag Phases – At the start of a cycle, left turn movements (phase 1 and 5) can lead, therefore lagging phases 2 and 6. However, 1 and 5 could also be the lag phases depending on how the signal is set up. The Lag phase information states which phases lag for the cycle.

Signal timing sheets have a whole slew of other information, above is just a sampling of some basic ones to get become familiar with this information.

Traffic Light Synchronization

On streets with multiple intersections in a row, it helps to have the lights coordinatedin such a way to have vehicles approach all green lights at each intersection. Trying to have coordination in all directions is very difficult, but for the larger streets that carry more through traffic, it’s feasible to have these movements coordinated. This keeps traffic flowing along the major arterial streets with minimal stopping, breaking and emissions produced. Synchronizing lights, while costly, is very beneficial to drivers making a trip through a series of the lights along a street segment.

Synchronized traffic intersections will have an offset value on the signal timing charts for the traffic engineers to use when programming the traffic signals. This offset value is the number of seconds it takes for a car traveling from being stopped at one intersection to the subsequent intersection down the road. This travel time allows the cycle length of the two intersections to be offset by a specific amount of time to allow cars to travel between the two intersections without stopping for a red light. Effectively creating a "green wave" for drivers.

Synchronizing lights can become messy very quickly depending on the streets being analyzed. To help, computer software is available to optimize signal timing most efficiently. Synchro is one common traffic software that calculates street signals to have the optimum signal timing values based on the inputted data. Synchro requires the user to input the existing roadway design, geometry, traffic volumes (number of cars), among other data, to have it calculate the most efficient signal timing values, such as cycle length, green times, etc.


Collecting data for inputting into Synchro requires vehicle counts to be collected. In it’s current state today, collecting traffic counts is a very time consuming and manually driven process. Because this is a time consuming and expensive process, updating traffic signals for synchronization only occurs once every few years (or more). This creates excess traffic congestion when travel patterns change on arterial streets. Traffic signal coordination does not have to be like this however, it can be an automatic, easy to maintain and provide peak performance to all users. In the next section we look at how this could be a serious possibility.

Smart Signal Technology

At fully-actuated intersections, street embedded wire inductive loops provide a sensor technology to have cars signal their presence in a specific lane. As an example, inductive loops in the left turn lane would signal to the traffic cabinet the left turn signal needs to be cycled through. If the loop does not detect a vehicle, it would skip that phase during the cycle to save time. These inductive loops at intersections however are wired differently then inductive loops on highways, in that they are all pulled together for each movement (not individual lanes) and the computer in the traffic cabinet only detects a vehicle in binary form (vehicle present or not present). Along the side of the intersection is a pull box where all these wires are reduced down to each of their traffic movements (left turn, through, right turn). Any vehicle counts are not obtained, as these sensors only function as dummy “on/off” switches.

In the future, more advanced technology could change this current design of intersections to allow for “smart” data collection to occur automatically every second of the day. By using the loops in the ground to collect vehicle counts, costs can be reduced for the required signal optimization data. Vehicle counts and signal optimization can be done on the fly or every 24 or 48 hours if necessary. This time consuming and high cost operation today will be completely automated down the road.

Of course this all assumes we will still be using human driven cars. With the current advancement from Google of autonomous cars, signalized intersections as we know it today wouldn't be necessary as the vehicles could navigate through a joining of two perpendicular streets by calculating each car's trajectory in advanced. This type of travel would be truly remarkable, of course we probably not even care because we would looking down at a tablet doing a Google search or some other mind stretching task.

To conclude, Intersections have come a long way from a 4 way dirt road junction, however with improved technology and acceptance by cities, intersections can continue to be safer, offer less dwell time and save emissions. If your looking for more information about traffic signal phasing and traffic Cabinets check out the How Traffic Signals Work Blog.


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This blog is to allow for case studys to begin studying traffic.

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