Yellow Lights vs. Safety Score

TL;DR; Some yellow lights are out there, just waiting to drop your Tesla Safety Score. This unfair situation is rare but hard (or impossible) to handle. At least as a driver, one can minimize the unfairness to some extent, primarily by driving slower, but also by anticipating the change.

You experienced it: you are chilling along in your Tesla and the light ahead turns yellow. Your first alarming thought is: “Ugh, there goes my Safety Score!”. You need to decide quickly! Should you brake hard or should you accelerate and breeze through the yellow?

We all suspect that “if the timing is just right”, we have only bad choices: Continue and run the red, speed up and run the yellow, or brake hard and damage the Safety Score. I shall call this the unfair zone. And it exists, but it’s quite narrow if you drive defensively.

Quick Analysis

For our analysis we introduce two virtual lines in front of the traffic lights.

  • If you passed the Go-Line is you can continue at your speed and make it to the intersection on yellow.
  • If you haven’t passed the Stop-Line yet, you can safely stop before the intersection. This is similar to the stopping distance they taught you in driver education, but here we use safe braking, not maximum possible.

Sometimes, these two lines are at the same location. Before the lines, you stop, after the lines, you continue.

We can’t paint these lines on the ground, as they depend on the speed you are driving, how hard you brake, and the timing of the yellow light – the yellow interval.

Part of learning to drive is to properly predict where the Stop-Line is (more on this below).

In an ideal world, the Go-Line is before the Stop-Line, and if you are in-between the lines when the light turns yellow, you can either stop or go, and both will work out. The longer the yellow interval, the longer the safe zone is, the easier it is to brake or continue safely.

The faster you go, or the longer the yellow interval, the further away the Go-Line is from the intersection.

The faster you go, the further away the Stop-Line is. The stop line does not depend on the yellow interval duration. Increasing the yellow interval moves out the Go-Line only and increases the safe zone.

In an unfair world, the Go-Line is after the Stop-Line. If the light turns yellow while you are in-between, you can neither make it, nor stop safely. This is the unfair zone.

This is not a new problem: it’s how a town can make a lot of revenue with red light cameras. Anyone caught between the lines can be ticketed, for hard braking or red-light violations.

Tesla drivers who care about safety score are the latest victims of the unfair zone.

There obviously is a minimum yellow interval that moves us into the ideal world. California defines minimal yellow intervals as follows (this table is simplified). This interval results in lines at the same location, good enough for a perfect human. A longer yellow interval creates a safety zone and a shorter interval creates an unfair zone.

Speed (mph) (85th percentile)Minimum Yellow Interval (s)
253
303.2
353.6
403.9
454.3
504.7
555
605.4
655.8
California minimal yellow intervals for 85th percentile speeds [1]

There is one small catch here. California computes its minimal Yellow Traffic Lights assuming you brake at 10ft/sec2. Unfortunately for Tesla drivers, that translates to 0.31g, and Tesla safety score will penalize you for that. You cannot brake hareder than 0.3g. Thus CA minimum intervals are unfair for Tesla drivers. They have to be about 0.1s longer to satisfy Tesla Safety Score maximum hard braking (0.2 for 60+mph), or Tesla increases their hard braking limit to 0.31g. After all, AP will brake harder than 0.3g, too.

Another catch is that California assumes a perception and reaction time of 1s. That is rather short, and only achievable if you are paying close attention.

Still, if the light is yellow for 0.1s or more longer than the California standard, then it’s possible to brake safely (without Safety Score penalty), or continue through without a red light violation. Hard braking becomes a driver skill problem, and not a safety score or yellow light problem.

In reality, it’s really hard to brake at exactly 0.3g when the light is exactly at the minimum yellow interval (+0.1s), so maybe we need to accept that human drivers cannot do it in such situations.

Avoiding Hard Braking at Yellow Lights

So let’s assume the yellow interval is slightly longer than the minimum for the speed of cars at that light. How can you keep your Safety Score high?

  • Drive in the 85th percentile. Lights are designed towards that. If you are in the fastest 15%, you are creating your own unfair zone. Slowing down always improves your safety zone, so if there are any doubts, slow down.
  • Never approach above 60mph, as the light is likely capped at 6s. Such speeds are just asking for an unfair zone.
  • Slow down when approaching intersections. It’s a lot safer than just breezing through.
  • Plan ahead. As you approach the traffic light, you keep repeating “stop, stop, stop, …” in your head until you reach the Stop Line. After that you can no longer stop safely, and you repeat “go, go, go, …”.
    This way, if the light turns yellow on your approach, you already know what to do. If the yellow light comes as a surprise, then you waste valuable time for perception.
    As you keep practicing, you learn from your mistakes, If you had to brake too hard, your stop line was too close.
  • You can “cheat” by turning on Autopilot. It will stop faster than you are allowed by Safety Score. But be careful, sometimes Autopilot will not engage.
  • Don’t forget that occasional hard braking only has a small impact on your safety score and you can dilute it. Don’t become upset if a yellow light gets you.

In case you are curious, here are the distances you have to stop depending on the speed:

Speed (mph)Speed (kmh)Stop Line (ft)Stop line (m)
25401132
30481444
35561957
40642472
45722989
508035107
558842127
609749149
6510557172
Stopping Distance at various speeds without triggering Tesla Safety Score Hard Braking penalties.
(These are longer than the more common stopping distances achieved with a hard brake that triggers ABS)

Fine Print

There are a lot of simplifications in the calculations above.

  • Humans aren’t perfect. Nobody can brake exactly at 0.3g. In reality, you will brake less, because going over will penalize you. Or you use regen, which is less, but by an unknown amount.
  • We ignore the length of the car. This matters because for stopping, you need to stop before the line, where as for going, you need to pass the line completely before it turns red.
  • If you are at the Go-Line, you can make it at the speed limit. In many intersections, that speed limit is not safe. But if that is the case, you should have approached at that lower speed.
  • Perception time. You should approach the intersection fully alert, and ready to stop, which will reduce your perception time. Still, the California standard expects 1s. That’s quite quick.
  • Yellow Intervals are capped at 6s, the speed for 60mph. If you go faster than 60mph, you are virtually guaranteed an unfair zone.
  • This is CA guidance, and other states may vary. And it’s just guidance. “Practitioners should exercise engineering judgment” and grade, geometrics, traffic behavior all factor in. The only rule is “should be between 3 and 6 seconds”

Sources
[1] California Manual on Uniform Traffic Control Devices (2014 ). Table 4D-102 (CA), page 935.

Last Updated on May 24, 2022.