Threshold speed for stale red light - Fuel Economy, Hypermiling, EcoModding News and Forum

prr · 03-03-2017, 10:52 AM

Every time I cruise up to a stale red light, I try to brake a bit early, allowing me to drive slowly, so that when it turns green, I'm still driving 5-20 MPH. If I stay off the brakes altogether, I find often that I have to brake hard at the last minute, which then brings my speed to (near) zero.

Is there a chart for how much energy you save, by not having to start your car from a full stop? I can only assume that the faster you are going, the less energy it would take. But is there a point that I definitely want to avoid--say, 5 MPH, or 15 MPH--at which point, there is little benefit to cruising at that speed, over starting from a full stop? If I knew what speed was my minimum, I would be better able to time my braking (or not brake).

Sorry if this isn't clear. I can explain in more detail if I need to.

Daox · 03-03-2017, 11:28 AM

Hey Prr, interesting question. Basically the answer is you want to maintain as much speed as possible. Every 1 mph you slow down is 1 mph you have to spend fuel to speed back up. There is no minimum threshold you want to avoid other than stopping completely. The higher speeds are more 'fuel costly' to regain only due to additional aerodynamic drag. Ignoring aerodynamics, 1 mph is 1 mph. Accelerating from 10-20 mph requires virtually the same amount of energy as accelerating from 40-50 mph. Some frictional losses are increased at higher speeds, but its negligible.

prr · 03-03-2017, 11:35 AM

OK, well that answers my question. I will just keep guesstimating, based on how stale I think the light is, how fast I'm going, and how much space there is for me to brake.

OK well at least I don't have to wonder now. Thanks.

Stubby79 · 03-03-2017, 11:50 AM

What Daxo said.

as far as I'm concerned, it's a pulse & glide situation, with the hopes of avoiding having to use the brakes. (Unless you have regen braking...then you only lose out on whatever inefficiencies there are in the system. )

Mr. Pancake · 03-03-2017, 01:24 PM

On my commute, at the lights that are likely to cause me to stop, I count to see how long it takes for the light to cycle. I use this to help judge where I need to let off the gas/how fast I should be going to not have to stop. You should always be trying to maintain as much momentum as you safely/legally can.

Don't be surprised if some people are annoyed by the fact that you are coasting up to a red rather than trying to get there first.

ThermionicScott · 03-03-2017, 01:36 PM

If no one is behind me, I'll start my coastdown really early when I see a yellow or red ahead.

If I misjudge it and the light turns green before I reach it, and so have to accelerate a little to get back up to speed, so be it. Better than getting going from a dead stop!

oldtamiyaphile · 03-03-2017, 08:12 PM

Wait until you guys get factory stop/ start cars.

Then you have a choice between braking early and maintaining momentum, or braking late and having the engine switch off sooner.

cliffordlj · 06-21-2017, 12:41 AM

As far as what daox said, 1 mph at 5 is not the same as 1 at 40 mph. The kenetic energy scales with the square of the speed. You can illustrate this by coasting up a hill and observing the vertical displacement will be significantly more as you slow from 40 to 20 as it will be from 20 to 0. However if driving a manual anyways keeping the speed above your first gear idle rpm will reduce clutch wear which is good

redpoint5 · 06-21-2017, 01:35 PM

The juggling act is accurately predicting when the light will change and the cars in front will begin moving. Brake too much, and the light will change before you get there, and you will have to accelerate, and possibly cause people behind you to miss the green light. Brake too late (or too soft), and you might end up stopping at the light.

If you have a very good idea of when the light will change, it's most efficient to brake very hard early on, reducing your speed to what will perfectly coast you to the light as it changes.

Be mindful of those behind you though. Most people are in a race to the red light, and won't be prepared for "some whacko nutjob" who brakes early and coasts to the red light.