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Written by: Ju Young Lee
Written on: December 10th, 2014
Tags: civil engineering, electrical engineering, transportation
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About the Author
Ju Young Lee is a computer engineering student in the class of 2015 at the Viterbi School of Engineering. She hopes to use her computer engineering background to create applications in healthcare.
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Volume XVI Issue II > Car Turns Signals: Why They Blink, Make Sounds, and Look a Certain Way.
Although car turn signals seem like a trivial part of the car, they are a very important communication tool that drivers depend on. Each design decision of the turn signals from the frequency of the blinking, the sound of the ticking, the color of the lights, and their location on the car are carefully engineered to ensure the fastest response time for drivers to recognize other drivers’ intentions.


You are driving down a street and you need to make a left turn. You move to the left most lane and ignite the left turn signal. Then, you have to wait for what feels like forever for the traffic light to allow you to turn. During this long minute, have you ever looked for a different car whose car signals blink at the same rate as yours? Or, ever wondered why it makes that tick tock sound that we are all so familiar with now? Starting a signal is something that drivers do every day. As trivial as the turn signal can be, compared to other larger car component such as brakes, the engine, or air bag, it’s a very important communication tool for drivers to indicate their intentions.
According to the Society of Automotive Engineers (SAE), over 25% drivers neglect to use the turn signal when turning, and 48% ignore it when changing lanes [1]. As a direct result of this issue, they report over 2 million crashes. This is a huge portion of crashes compared to the estimated 950,000 crashes from distracted driving [1].


After the establishment of the National Highway Traffic Safety Administration (NHTSA) in the 1960s, car turn signal lamps were required for all vehicles [2]. However, inventors began to patent turn signal devices in the early 1900s. In 1909, a British man named Percy Douglas-Hamilton patented a set of mechanical hands that were attached on the sides of the car and illuminated when you wanted to signal a turn. Oscar J. Simler, in 1929, created a signal of his own which had a four-lobed shape with lenses for lights indicating slow, stop, left or right turn (Fig. 1) [3].
[image=1294 file="null" placement="center"]F​igure 1: Oscar J Simler’s handmade turn signal.[/image]
It was not until the late 30’s that a man named Joseph Bell patented the first device that flashed. And, then in 1939, Buick, the American car manufacturer, introduced turn signals as a standard feature for cars. Other car manufacturers followed Buick after they added the self-canceling mechanism, including Cadillac, LaSalle and Hudson Country Club. Many companies would advertise it as an optional feature to add like how we would add a GPS navigator today. Chevrolet, Oldsmobile, and Pontiac would offer the turn signal option for an additional cost of $7.95 in the 1940s. As more companies began to adopt it as a feature, turn signals were viewed as a luxurious option, in par with adding more powerful engines or two-tone paint [3].
Although the official law requiring turn signals was adopted in 1960, it took more than a decade to establish the details, which many people are still debating. SAE produced multiple reports throughout the 60s detailing the specifications for car turn signals, such as amber lens's for front turn signals while rear signals could be red or amber, lit lens area had a minimum requirement of 50 cm2 [4], and a range of 60-120 flashes per minute [5]. The SAE took these matters seriously because they viewed the turn signals as an information-transmit​ting system that delivered valuable information from driver to a driver.

Amber or Red?

It is estimated that more than 90% of the sensory input to a driver is obtained visually [6], thus choosing the correct specification for the turn signal lamp is imperative. Lights in cars and other warning lights like ambulance sirens use a spectrum of red as the color. Human vision peaks in the yellow-green portion of the spectrum, with white as the most visible color, followed by green, amber and red [6]. White is effective in getting attention but drivers would have a hard time identifying the vehicle itself. Green is the next visually effective but has a connotation of meaning “go” or “safe”, which is not useful for indicating stops and warning signs. Thus, yellow and red have become the popular colors for warning and caution.
There is an ongoing debate of whether the rear light color should be amber or red. Although it seems like an insignificant matter, the NHTSA in 2009 released a report that amber rear turn signals are 5.3% more effective than red turn signals at preventing rear-end crashes [5]. In addition, other countries like the UK, Australia, Japan, and China have set rear signals to be amber color as the regulation. However, the United States in 1970 released the Federal Motor Vehicle Safety Standard No.108 allowing rear turn signals to be either red or amber in color.
Amber light is not cost effective, and thus requires a big change in the rear light design. Car manufacturers have been going back and forth, selecting red for one model then amber the next model. Nissan Altima from 1998 to 2006 switched their color four times [5].
The proponents of red color argue for the cost effectiveness and simplicity of the design to use the same color for displaying the different information. In some of the older designs, instead of having a separate turn signal lamp, the brake red lamp is used for all the functions; it’s a steady dim for tail, steady bright for brake, and flashing bright for the turn signals. This is due to the requirement of each lit lens area to be at a minimum 50 cm2 [5]. In the 1950s, SAE lighting committee calculated the most effective luminous lens area. Among other factors, it was to ensure that the bulb when overheated would not crack the lens. By placing this parameter that required the 50 cm2 area to be all lit, it forced a certain distance from the bulb to the lens so that the 1950s non-heatproof plastic lens wouldn’t crack [5]. This meant that having multiple lenses for different functions could make the rear look clustered and not appealing. Thus, the brake light was used to communicate all functions.
However, in the NHTSA report, having amber light proved to be better. Furthermore, we have better hardware to not worry about lenses cracking. When you simultaneously brake and signal, it yields two-thirds of the full brake lamp indication. Volunteers of the case study took longer to recognize that the car ahead was signaling a turn because it was also braking at the same time [5].
In other designs, the car’s turn signals and brake lights are separated. However, with the red on red next to each other, it still resulted in a slower response time than having amber turn signals and red brake lights. While ECE International regulations require brake light and the rear fog light be at least four inches apart, there is no separation requirement for brake lights and turn signals in the U.S. [5]. Different color and separation of rear lights is beneficial if it means that drivers have a quicker response time.

Spatial Configuration: Medially vs laterally placed indicator

In the discussion on red or amber lights, we see how engineering and design choices conflict. However at the end, it is the duty of the engineers to mandate these details for the safety of the public. And, the designers must abide by these rules and work within the constraints. Similarly, it has been shown through case studies that spatial configuration of the rear turn signal is very important.
According to a study at the Centre for Cognitive Neuroscience at the University of Wales, the participants’ errors and response time was quicker when the signal position relative to the headlight was compatible with the correct answers [7]. That is, if indicating a left turn, the left turn signal should be placed on the left side of the headlight as shown in the top image. Thus, laterally-placed indicators, meaning lights placed on the outer side, create a faster response time than medially placed indicators, which are placed on the inner side. The figure below shows the subtle yet important differences in the indicator placements (Fig. 2).
[image=1341 file="null" placement="center"]F​igure 2: top image is laterally placed, bottom image is medially placed .[/image]While the case study was conducted in a controlled setting, in real situations with other distractions, it can only exacerbate the response time slower if car designs place signals medially. A slower response time to realizing the other cars’ decision to switch lanes creates a more likely chance of car accidents.

Frequency of the signal

The Society of Automotive Engineers, in 1965, established that the car turn signal frequency should be between 60-120 flashes per minute [9]. This parameter was extensively calculated and supported by other case studies where participants were exposed to different frequency and percent on-time of the signal. (Percent on-time refers to what percentage of a cycle, the time from signal is on state verses the off state.) So if on-time is 85 percent, signal is on for 85% and off for 15% of the whole cycle [9]. The table below shows the rating that the participants have evaluated based on how conspicuous the signals were (Fig. 3). The box in the middle
is the SAE standard which shows the SAE standard. The dotted area shows other parameters that produced similar satisfactory rates as those of SAE standards.
[image=1342 file="null" placement="center"]F​igure 3: Chart of SAE Standards and parameters.[/image]F​rom this study, higher flash frequency and minimum flash duration (less on-time percentage) provided the best conspicuity and shorted response time. Since the driver is not always attending to the rear lights of the car in front of them, the parameter that gets the driver’s attention the fastest is the best one. Having a few second advantage can be trivial but when you are driving at 60mph, the shorter response times mean recognizing that the front car is changing lanes few yards earlier.

The Ticking Sound

The ticking sound acts as a reminder for the driver that the signal is on so we can turn it off when we are not using it. Although there is a self-canceling mechanism for turning, drivers need to be conscious of the noise when switching lanes. Without the indicator sound, it can give false information for other drivers on the road.
The ticking sound that we all are familiar is a side effect of the first circuit design to blink on and off periodically. In fact, current circuit mechanism uses an electronic flasher compared to the older thermal flasher which doesn’t produce this sound. However, because we became so used to this sound, they are artificially produced and played on the speakers.

Thermal flasher- older model

The first signal flashing mechanism used a thermal flasher, which consisted of a bimetallic strip that bent and straightened when heated by the electron current (Fig. 4).
[image=1344 file="null" placement="center"]F​igure 4: The structure of a bimetallic strip.[/image]The bimetallic strip consists of two metals with different heat capacity stuck together. When the car turn signal is either pushed down or up to signal left or right, the circuit is connected, producing a current to flow through the bimetallic strip. Due to the different heat capacity, one metal bends more than the other and causes the strip to bend a certain direction. This bent strip now makes contact with a metal allowing the electron current to go through the circuit and light the bulb [10]. When the strip cools off, it goes back to its original state; there is then no contact, thus turning off the light bulb. This mechanism acts as a switch and the time it takes to heat and cool is the frequency of the signal [10]. The strip making contact by bending and straightening is the tick tock noise that we are all familiar with.
When one of the light bulbs goes out, the circuit is designed so that the other light bulb will blink at a noticeably slower frequency. It takes a longer time to heat the strip to bend because less current is flowing through the bimetallic strip .
The thermal flasher has limitations due to its dependency on the bending of the metal strip, which wears down and cracks. Thus, engineers introduced an electronic flasher that doesn’t depend on a heat source but rather a magnetic field to manage the switch. Sending current through a coil creates a magnetic field with a force to pull and push the necessary component to complete the circuit and light the lamp (Fig. 5) [11].
[image=1345 file="image5.png" placement="center"]F​igure 5: Complete circuit of the light and the lamp.[/image]By relying on electronic circuitry and not heat, extreme climates are not a factor. The thermal flasher does not wear out as fast as the metallic strip. The circuit is designed so that when a light bulb is out, the other one blinks at twice the normal speed. In the previous finding on signal frequency, higher frequency is more noticeable than a slower frequency. The driver can detect the change faster and fix the lamp.

Future Development

Because the electronic circuit doesn’t produce the tick-tock sound, some manufacturers have voiced ideas on customizing the signal sound or even getting rid of it with a smarter technology to automate the signal canceling even for lane changes. Lexus has options for drivers to control the volume of the turn signal and set the number of blinks [12]. The driver can specify their tun signal to only blink for a certain number of times and then have it automatically turn off.
Intelliturn has engineered a smart turn signal system for cars [13]. Using sensors that compute how much the car has turned and then applying machine learning, the smart turn signal system can detect when the car is switching lanes or turning. Through this detection, this smart system can notify drivers when they forget to use a turn signal from the previous left turn as well as apply the self-canceling feature for all functionalities. The technology is still in the early stages but this can be a very promising feature that will alert drivers to use car signals and prevent any premature shutting off of signals by drivers.
There are many new ideas to make their car safer, customizable, and appealing in design. Engineers and car manufacturers constantly work together so that with every feature, the safety of the public comes first.


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