What is a Time-Difference Signal?
Some traffic signals installed at intersections have signs that say "Time-Difference Type" or "Time-Difference Signal." Let's delve into what this means.
Overview
A time-difference signal is a traffic light designed to ease congestion and prevent right-turn accidents by extending the green light duration for one direction of traffic at an intersection, allowing right-turning vehicles to flow more smoothly against the opposing traffic flow.
In a standard time-difference signal, even after the opposing traffic turns red, the green light for the direction with many right-turning vehicles remains extended. This resolves traffic imbalances.
However, there are multiple types of time-difference systems. Additionally, the National Police Agency has issued guidelines to ensure that accidents do not occur.
The standard signal phasing for a time-difference signal is as follows.
Incidentally, to prevent accidents, time-difference signals are in principle only installed at T-junctions or intersections where right turns are prohibited in one direction.
Differences from the Right-Turn Separation System
The right-turn separation system (red light + right-turn arrow signal) is like a superior version of the time-difference signal, as it can handle situations where there are many right-turning vehicles from both directions of the road at an intersection. However, it has the drawback that it cannot be installed on narrow roads or intersections because a right-turn lane is required for installation.
Please refer to the following article for more details.
Regional Characteristics of Signboards
The signboards indicating a time-difference signal have characteristics that vary by prefecture. Broadly speaking, there are four ways of writing it: "Jisa-shiki" (Time-Difference Type), "Jisa Shingo" (Time-Difference Signal), "Jisa-shiki Shingo" (Time-Difference Type Signal), and "Jisa-shiki Shingoki" (Time-Difference Type Traffic Light). While horizontal signboards are common, there are also vertical ones and signboards with a checkered pattern.
Some examples are shown below.
(A square checkered "Time-Difference" sign on the signal arm)
("Time-Difference" below the signal)
("Time-Difference Signal" above the signal)
("Time-Difference Signal" below the signal)
National Police Agency Guidelines
The National Police Agency outlines the operational policy for time-difference signals in the guidelines revised in 2008 (Heisei 20), titled "Establishment of Operational Guidelines for Control via Time-Difference Signal Indications (Notification)."
The main points of these guidelines are summarized below.
● When installing a time-difference signal, it is mandatory to install a pedestrian signal at the crosswalk where right-turning vehicles will pass. This signal must be red during the time the green light is extended.
● A sign indicating that it is a time-difference signal must be installed. (See the figure below for the design)
● When installing a time-difference signal at a four-way intersection, a no-right-turn regulation must be implemented all day for traffic entering from the direction where the green light is shortened.
● If a no-right-turn measure cannot be implemented, control must be performed using a separate-phase method.
● The lead-start method at standard four-way intersections should not be used due to the risk of collisions between right-turning vehicles and oncoming straight-moving vehicles from the delayed-start direction, as well as the danger of vehicles in the delayed-start direction starting prematurely due to assumptions.
● All-direction arrow signals on the side where the display time is extended should not be used, as they have the same meaning as a green light and are likely to cause confusion for drivers.
Special Indication Methods
While many time-difference signal indication methods follow the National Police Agency guidelines, special indication methods also exist depending on the region or intersection. Here are some representative examples.
Lead-Start Method (Early-Start Method)
A method where one direction is allowed to proceed before both directions turn green is called the lead-start method (early-start method). In the case of the lead-start method, the sign often explicitly displays "Lead-Start."
(Mitaka City, Tokyo)
Common lead-start indication methods are as follows. In the case of the lead-start method, all-direction arrow signals are often used to explicitly indicate the timing when both directions turn green.
(Bottom) At a T-junction, the bottom-to-top all-direction arrow signal starts first, then both directions turn green.
(Ryogoku, Sumida Ward, Tokyo)
The National Police Agency guidelines mentioned earlier do not recommend the installation of the lead-start method at standard four-way intersections.
Please refer to the following article regarding lead-start time-difference signals.
Niigata Method
In Niigata Prefecture, there is a system where, during the extended green light period, a right-turn arrow signal is illuminated in addition to the green light. Compared to standard time-difference signals, this is easier to understand because an arrow signal is added during the green light extension; however, as it deviates from the National Police Agency's guidelines, its numbers have been decreasing in recent years, and it became extinct from public roads at the beginning of 2026.
Please refer to the following article for details on the Niigata-style time-difference signal.
Osaka-style arrow system
At some intersections in Osaka, Gifu, and Tochigi Prefectures, the extended green light phase is indicated by all-direction arrow signals. Compared to standard time-difference signals, this is easier to understand because the extended green light phase is replaced by arrow signals for the directions in which traffic can proceed, but it deviates from the National Police Agency's guidelines.
A system known as the so-called "Osaka-style arrow time-difference signal."
Separate output system
A system that provides independent "extension" indications for both the northbound and southbound directions. It is like a combination of a lead-start system and a standard time-difference signal.
One example of the separate output system is as follows. The 1-phase and 5-phase signals correspond to the green light extensions for the northbound and southbound directions, respectively.
A separate output system that provides independent indications for both the northbound and southbound directions
Please refer to the following article for details.
Auxiliary signal: Right-turn only signal
In large intersections in Tokyo and other areas where time-difference signals are installed, right-turn only signals are installed to compensate for the lack of clarity.