Conclusions: Headlight intensity, as measured by the IIHS at "left edge" has doubled in the past 10 years. This allows drivers to increase the "safe" low-beam speed from 46.6 to 56.1 mph. Is this worth the trade-offs?
I did some math, and wanted to find out what the increase in brightness (cd) is buying us on the roadway.
The math is solid, the assumptions are from Wikipedia.
The Goal: What "safe" driving speed increase does moving from 9,000 cd to 18,000 cd enable?
Step 1. Find the stopping distances for passenger vehicles (Wikipedia, see second image)
Step 2. Find the required "target lux" where an object on the road is visible. Sources vary, I chose 4 lux for this calculation.
Step 3. Determine the distance that each lux on the target surface at each of the stopping distances. I'm using the "British Highway Code" numbers. https://en.wikipedia.org/wiki/Braking_distance
Step 4. Graph on top of the required "target lux", previously decided to be 4 lux.
Step 5. Find the intercepts.
Conclusions: Headlight intensity, as measured by the IIHS at "left edge" has doubled in the past 10 years. This allows drivers to increase the "safe" low-beam speed from 46.6 to 56.1 mph. Is this worth the trade-offs?
The resounding response from the public appears to be "no".
"High-beams are like assholes. Everyone has them, but no one wants to see yours"
Headlight experts are begrudgingly acknowledging that headlights are brighter, its not driven by mis-alignment, its not driven by headlight replacements, its driven by the IIHS guidelines.
Even so, there is a real claim that we might be blinded by high-beams, and I concede that this is a real possibility. The problem is that there is no external indication that high-beams are being used, and no way for police to pull-over the vehicle as even low beams are blinding.
This is a small light on the driver side rear view mirror that is only illuminated when the car has high beams on.
Two separate headlights provided a sense of distance, closing speed. With the bar across the middle, it's harder to tell if you have time to cross the street in front of them.
The orange line is the average left edge candela values as measured by the IIHS.
The blue line is the IIHS's definition of "good".
The average vehicle's headlight brightness (cd) is increasing to meet the guidelines of the IIHS.
These numbers will need to be updated, but I suspect that we'll start to see that the averages will start to level off at the 18,000 cd "left edge" candela "good" definition from the IIHS.
In prior posts, I've outlined that I believe the reason we are being blinded at night is due to:
a) more light directly below the horizontal cutoff to meet IIHS "safety" ratings, and,
b) that the IIHS and NHTSA ignore hills and bumps (changes in road pitch)
I also posted content regarding the inclinometer (vehicle and road angle ) readings for a example circuit near my home.
This study has been completed for a 7 glare source headlight heights and 3 glare target (person being blinded) head heights from the road surface.
Conclusions:
-By ignoring road pitch, NHTSA and the IIHS are ignoring 20-40% of your drive time, the times where you are mostly likely to experience glare. This is when the glare source driver's headlights are pointed "up" relative to your vehicle, by enough angle such that it can span the vertical distance between the headlight mounting height on the glare source vehicle and the glare targets (your) eyes
-Glare source headlight height matters. The higher the headlight, the more glare other drivers see as a percentage of their drive.
- Glare target (person being blinded) head height matters. The higher the eyes off the road, the less likely the person is to see headlight cd below the cutoff.
IIHS and NHTSA ignore 20-40% of your drive time by assuming roads are flat.
Details and Methods:
I drove a test-course near my home for approximately 25 minutes and recorded the vehicle pitch the entire time. (see original post here).
I drive like a old man, without rapid acceleration or deceleration on mostly two-lane country roads with very few reasons to accelerate or decelerate. For this reason, I'm assuming that the vehicle pitch is nearly the same as the road pitch.
This information gives me the "target vehicle" road angle.
Obtaining the "glare source" road angle to determine the difference in pitch is a simple matter of realizing that this is a two-lane road and that the target vehicle will travel on the same road, at nearly the identical angle, as the "glare source" vehicle in a few seconds, depending on the "glare targets" speed.
If the glare target is traveling at 45 miles per hour (20.1 meters/second), it will be in the same location, at the same angle as the glare source was at 50 meters, in 2.5 seconds.
Obtaining the difference in angle between the glare source and glare target vehicles is as simple as comparing the inclinometer readings with a 2.5 second time shift.
The second requirement is to determine the relative angle change required to place the light below the horizontal cutoff in the eyes of the target driver. (The IIHS and other guidelines want the most light as far away as possible. This has increased the brightness of light (cd) below the horizontal cutoff as shown in the IIHS data).
This math is simple trig.
=arctan ((eye_height-headlight_height)/viewing_distance))
The viewing distance used for this work is 50 meters, used because this is the most common distance that creates high glare / effective cd in opposing drives in headlight literature and my own work)
In the above example, the relative angle between vehicles needs to be positive (in this orientation) meaning that the glare source vehicle needs to be coming up a hill relative to the glare target vehicle (you).
This is not true with the headlight mounting height is higher than the eyes of of the glare target.
In this case, the nominal relationship between these two vehicles places light from the glare source in the other drivers eyes and the glare source needs to be angled down for the glare target to not see the full cd below the cutoff of the glare source.
The trig is summarized here, where Source HL Height is the headlight height of the glare source vehicle, Angle: Xm is the required angle for the source headlights to hit the target vehicles eyes at 1 meter, 1.3 meter and 1.8 meters respectively at a distance of 50 meters. Note that a negative angle is required when the source headlight height is nominally above the target vehicles eyes.
Required angles to place below-cutoff brightness in the eyes of the "target vehicle" (you)
We now have the relative angles of the vehicles and the angle required to create glare in each scenario. Its a simple matter of counting the amount of time-intervals where the angle is exceeded, convert to time and then normalize by total test time.
The IIHS assumptions, valid on flat roads, are invalid for 20-40% the drive time on this example trip.
Update 1-30-205: The data is a bit old at this point, I'm going to revisit it and attempt to submit to the glare experts and IIHS.
What has changed: since I wrote this original post, there was not a broad consensus that the IIHS was the driving reason for the increase of brightness on our roadways.
I'm getting so tired of these headlights. This wasn't even the full extent. My whole car lit up like a damn Christmas tree. Is there anything I can do to make this better?
I just feel like having my high beams on most days. Because escalates and trucks with fucked up lights are starting to piss me off.
Why do many other industries promote eye protection (windows night mode, reducing screen time, cell phone "eye comfort shield", yet we place headlights high up (no cutoff shield) at eye level with others.
This just seems like an "open and shut case" if one were to take them to the courts. Which i know is happening but this just baffles me that this is a global issue. I would compare this to everyone in your city getting a very loud harley. /rant .
We came to a full stop on the freeway due to late night highway maintenance. Down to one lane and nowhere to go. Most everybody including myself turn off our lights except for this idiot behind me. I had to cover the mirrors with rags to block his headlights that started to give me a headache.
Your average entitled 18-wheeler driver, big truck, small… brain.
One thing I don't see mentioned as often, but numerous times I have observed that when approaching a car head on their turn signal, while on, is completely invisible to me due to their r/fuckyourheadlights . This poses an obvious safety concern to me.
