Wednesday, November 9, 2011

A cool and innovative speedometer... for 1939

Paul Leroux
Earlier this week, I referred you to a whitepaper written by my colleagues Scott Pennock and Andy Gryc. In the paper, Scott and Andy argue that driver distraction is not, in fact, a problem of distraction, but of situational awareness, or SA. Boost a person's SA, and you improve their ability to drive safely.

But how, exactly, do you improve SA? The paper discusses various techniques, and I couldn't possibly do justice to all of them here. But one approach is to supplement the driver's eyes and ears with indicators and warnings, based on information from sensors, roadside systems, and other vehicles.

Here's an example: A system in your car learns, through cloud-based traffic reports, that the road ahead is slick with ice. It also determines that you are driving much too fast for such conditions. The system immediately kicks into action, perhaps by warning you of the icy conditions or by telling you to ease off the accelerator.

Too bad the engineers who designed the 1939 Plymouth P8 didn't have access to such technology. I'm sure they would have embraced it totally.

You see, they too wanted to warn drivers about excess speed. Unfortunately, the technology of the time limited them to creating a primitive, one-size-fits-all solution — the safety speedometer.

Color coded for safety
From what I've read, these speedometers switch from green to amber to red, depending on the car's speed. I've only seen still photos of these speedometers, but allow me to invoke the magic of PhotoShop and reconstruct how I think they work.

The safety speedometer has a rotating bezel, and embedded in this bezel is a small glass bulb. At speeds from 0 to 30 mph, the bulb glows green:

At speeds from 30 to 50 mph, the bulb turns amber:

And at over 60 mph, the bulb turns red:

Given the limitations of 1939 technology, the Plymouth safety speedometer couldn't take driving conditions or the current speed limit into account. It glowed amber at 30 mph, regardless of whether you were cruising through your neighborhood or poking down the highway. As a result, it was more of a novelty than anything else. In fact, I wonder if people driving the car for the first time would have focused more on watching the colors change than on the road ahead. If so, the speedometer may have actually reduce situational awareness. Oops!

Compare this to a software-based digital speedometer, which could take input from multiple sources, both within and outside the car, to provide feedback that dynamically changes with driving conditions. For instance, a digital speedometer could acquire the current speed limit from a navigation database and, if the car is exceeding that limit, remind the driver that they risk a speeding ticket.

That said, I have a soft spot for anyone who is (or was) ahead of their time. Some enterprising Plymouth engineers in the 30s realized that, with faster speeds, comes the need for even greater situational awareness. Their solution was primitive but it offered a hint of what, more than 75 years later, can finally become reality.


  1. Yes, it was pretty cool for its time. I own a 1940 plymouth and my speedometer is similar to the one above. It's interesting how it works. It doesn't actually 'glow' as if powered by an electrical bulb. What it is is that underneath the bezel, there are colors painted on the actual plastic backplate of the speedometer hidden from the eye by the 'floating' bezel. It's painted green from 0-30; from 30-50 it's painted amber and from 50-100 it's painted red. So, when the bezel moves indicating the speed, the glass bulb 'floats' over that painted area and you can see the color through the clear glass bulb. It's pretty cool, but the green is actually harder to see than the amber and red. Those two colors show clearer, especially at night when the dash lights of the speeodomer are lit. The green for some reason doesn't show as well.

  2. Anonymous, thanks for much for sharing -- so cool to find out how this actually worked. I wonder whether the green color has faded over the years -- or maybe not :-). Have a great weekend,
    - Paul