A typical use of an overlap is where you have a dedicated right turn lane, that you want to signalize with a right turn arrow. That right turn may be able to operate when the adjacent thru movement is green, but also when the corresponding side street left turn is green. The signal overlap allows the right turn traffic to continue flowing, when the adjacent thru traffic is going, and to continue when the signal stops the adjacent thru traffic to serve the side street left turn traffic. Care needs to be taken to make sure that the protected right turn arrow overlap does not create a problem for pedestrians.
The use of an overlap can help the overall traffic progression through the intersection (emphasis on "can").
So why not do this everywhere?
In order to implement this type of overlap, you need to have some specific things in place.
You need a signalized intersection, that has an dedicated right turn lane, plus the side street traffic must have a corresponding protected left turn lane. Protected left turn signal operations can easily be less efficient than protected / permissive, or straight permissive operations. Extra right turn lanes take extra land, which costs more to purchase, costs more asphalt (which costs more to construct and more to mitigate stormwater for), requires bigger traffic signal poles and mastarms (more $$ again) and almost always increases the size of the intersection - which directly affects the signal timing parameters.
Like anything, it has pros and cons. Where applicable, the right turn overlap may be a really good idea, if (a) you can afford it, (b) you can make the lane long enough to extend beyond the back of the queue of thru vehicles, and (c) it makes sense from a pedestrian and vehicle perspective.
Negative Pedestrian Overlaps
It is important to consider how the overlap will operate when there is a pedestrian movement adjacent. For instance, if the right turn overlap is to operate with the northbound thru movement, and the westbound left turn movement, the overlap needs to not conflict with a pedestrian movement operating at the same time as the northbound thru movement. You don't want the right turn green arrow to operate across a pedestrian walk / don't walk as this would be a conflicting movement.
Some traffic signal equipment is capable of operating a "negative pedestrian overlap", or a "minus pedestrian overlap", which allows the traffic signal equipment to process the presence of a conflicting pedestrian movement, and provide different vehicle indications based on what is going on with the pedestrian indications.
Several traffic signal controllers currently being sold now don't have the ability to operate in a minus ped overlap operation. Several manufacturers have traffic signals that state that they do, but in actuality, there are some quirky things in the operation.
Some agencies implement a quasi overlap type of operation, by directly connecting the right turn green and yellow arrow electrical wires to the side street left turn protected movement, and providing a green ball to the right turn movement when the signal provides a green ball to the adjacent thru movements. This can be a little herky-jerky, as the traffic in the right turn lane can observe their green ball indication go from green ball, to yellow ball, to red ball (same as the thru movement), then after the end of the all-red clearance interval, the right turn indication would then turn to a right turn green arrow when the side street left was provided a left turn green arrow.
Some controller manufacturers can have unexpected, problematic issues with minus ped right turn overlaps. Testing of the current version of controller firmware is essential. Also, it is essential that the conflict monitor / malfunction monitor unit in the cabinet must monitor for the vehicle yellows against the pedestrian WALK / flashing DONT WALK. Some controllers are loose when they apply the pedestrian call, causing problems.
Generally, controllers decide what phase to operate next immediately prior to the beginning of the yellow for the current phase. Some controllers will allow for a late pedestrian call to be registered which can cause a conflict. One controller with an official firmware version offered by the (un-named) manufacturer that I use actually will accept a pedestrian call after the phase next has been selected. This means that if the pedestrian call is placed in the last second or two of the all-red phase of the thru movement, the controller will serve the pedestrian movement with the onset of the associated green phase, while simultaneously timing down the yellow for the adjacent overlap. Essentially, this is a technical way of saying that the controller software allows for the controller to create a conflict that is only evident if you are paying attention, while simultaneously displaying a pedestrian WALK plus a conflicting right turn yellow arrow. This is a conflict. We caught it because our monitors are all set to monitor the presence and absence of all green, yellow, red, WALK flashing DONT WALK and steady DONT WALK indications. We turned on four signals in four days, with the official software, and found that we had signals going into all-red flash every couple of hours.
Other Uses For Overlaps
Traffic signal overlaps are used in some subtle ways.
At some freeway interchanges (typically Single Point Urban Interchanges, or SPUI's) they can be used to help clear the huge intersections more efficiently. In some SPUI's, there are two sets of signal heads for several movements. The leading signal heads are driven by the overlaps, the signal heads inside the intersection are driven by the phase movements. When used with Optically Programmed Signal Heads, the use of overlaps allow the approaching driver to be shown that the signal is transitioning from green to yellow, while the driver within the intersection would be shown a different length of yellow and all-red. Subtle, but sometimes important.
For Tight Diamond Interchanges (TDI), sometimes a single traffic signal controller controls both traffic signals at the intersection. The use of overlaps allows the signal to provide a red to the first signal you approach, while continuing to flush the next signal with a green. Once again, subtle, but rather important to keep the intersection from going to gridlock.
Overlaps also allow for some pretty complex operations at traffic signals.
One case that I recently designed and turned on, we had a "T" intersection with a slip lane on one approach. We wanted to keep the traffic moving on the slip lane, and be able to allow the signal to select the other movements as necessary. The extensive use of overlaps in the signal operation allows the signal to maintain the slip lane, and choose alternating lefts and rights while keeping the signal in the NEMA Dual 8-phase quad operation, which is the only way that this particular controller likes to operate in coordination.
In another case, I designed a signal where there was one controller / cabinet operating two closely spaced intersections. By using overlaps, I was able to get each of the signal indications for the nearside intersection's thru movement to go yellow by a user specified amount of time prior to the far side signal going yellow. This created a situation where cars were not stranded in between the signalized intersections. Likewise, when the signals went green, the far intersection's signal went green prior to the near intersection's green to get the traffic moving more efficiently.
Examples of phasing diagrams with overlaps are as follows:
The following signal has right turn overlaps on each approach. The overlaps conflict with potential pedestrian movements, so the phasing diagram specifically calls out the traffic signal operation.
