# Weird highway sign. **URL:** \ **Category:** Factual Questions\ **Created:** [June 6, 2006, 2:10am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566 "2006-06-06T02:10:17Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Drum\_God](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drum_god/32/44_2.png) [@Drum\_God](https://boards.straightdope.com/u/Drum_God)\ **Post date:** [June 6, 2006, 2:10am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/1 "2006-06-06T02:10:17Z")
Today, I drove into Elgin, Texas and passed a sign at the city limits. It said “Engine Braking Prohibited - City Ordinance”. Engine braking, as I understand it, is to slow or stop a large vehicle by downshifting rather than pressing the brakes. This is especially effective when going down hill. Elgin has no hills. It does, however, have stoplights. Why would the City of Elgin care about engine braking? How would they enforce such an ordinance, anyway? ---
**Author:** ![elmwood](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/elmwood/32/3060_2.png) [@elmwood](https://boards.straightdope.com/u/elmwood)\ **Post date:** [June 6, 2006, 2:12am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/2 "2006-06-06T02:12:33Z")
[The Master speaks](http://www.straightdope.com/mailbag/mjake.html), as he eventually will on every subject known and unknown to man. ---
**Author:** ![AWB](https://avatars.discourse-cdn.com/v4/letter/a/bb73d2/32.png) [@AWB](https://boards.straightdope.com/u/AWB)\ **Post date:** [June 6, 2006, 2:15am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/3 "2006-06-06T02:15:03Z")
> [@Drum God](#): > > Today, I drove into Elgin, Texas and passed a sign at the city limits. It said “Engine Braking Prohibited - City Ordinance”. Engine braking, as I understand it, is to slow or stop a large vehicle by downshifting rather than pressing the brakes. This is especially effective when going down hill. Elgin has no hills. It does, however, have stoplights. Why would the City of Elgin care about engine braking? How would they enforce such an ordinance, anyway? Engine brakes are brakes that slow the engine down directly, just like ordinary brakes slow the wheels directly. (It’s like a big disc brake, only mounted to the engine.) They make a helluva noise, which municipalities sometimes seek to eliminate by outlawing the use of engine brakes within their limits. Alexandria, VA is a city in my neck of the woods that has a similar sign at their city limit. ---
**Author:** ![crowmanyclouds](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crowmanyclouds/32/19884_2.png) [@crowmanyclouds](https://boards.straightdope.com/u/crowmanyclouds)\ **Post date:** [June 6, 2006, 2:15am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/4 "2006-06-06T02:15:45Z")
[What is “engine braking” on 18-wheelers and why is it banned everywhere?](http://boards.straightdope.com/sdmb/showthread.php?t=342475) CMC fnord! ---
**Author:** ![Airman\_Doors\_USAF](https://avatars.discourse-cdn.com/v4/letter/a/e36b37/32.png) [@Airman\_Doors\_USAF](https://boards.straightdope.com/u/Airman_Doors_USAF)\ **Post date:** [June 6, 2006, 2:19am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/5 "2006-06-06T02:19:57Z")
They are also known as Jake Brakes, although that is a brand name co-opted to mean compression brakes in general (in the same way that Xerox refers to all copiers). [Here](http://en.wikipedia.org/wiki/Jake_brake) is some more information on compression braking. There are other links on that page, but the link has the information you wanted to know. ---
**Author:** ![Airman\_Doors\_USAF](https://avatars.discourse-cdn.com/v4/letter/a/e36b37/32.png) [@Airman\_Doors\_USAF](https://boards.straightdope.com/u/Airman_Doors_USAF)\ **Post date:** [June 6, 2006, 2:23am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/6 "2006-06-06T02:23:26Z")
> [@AWB](#): > > Engine brakes are brakes that slow the engine down directly, just like ordinary brakes slow the wheels directly. (It’s like a big disc brake, only mounted to the engine.) There’s no “engine brake” per se. The noise is a byproduct of the valves and the compressed air in the pistons working to slow the engine down. ---
**Author:** ![Airman\_Doors\_USAF](https://avatars.discourse-cdn.com/v4/letter/a/e36b37/32.png) [@Airman\_Doors\_USAF](https://boards.straightdope.com/u/Airman_Doors_USAF)\ **Post date:** [June 6, 2006, 2:24am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/7 "2006-06-06T02:24:54Z")
Cylinders, that is, not pistons. ---
**Author:** ![KeithT](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@KeithT](https://boards.straightdope.com/u/KeithT)\ **Post date:** [June 6, 2006, 2:27am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/8 "2006-06-06T02:27:07Z")
As other posters have mentioned, engine braking uses the compression of the engine, rather than wheel brakes, to slow the truck. When engine braking is turned on, the fuel injectors turn off, and the exhaust valves open early, at the top of the compression stroke. Since this involves compressing air and then venting it quickly, it’s incredibly loud. (Take it from someone who lived next to a major interchange for half a year. _Why would the City of Elgin care about engine braking?_ Presumably, at some point in the past, truckers developed a habit of using engine brakes, rather than wheel brakes, when they slowed down for the stoplight. Why? I’m not sure. I imagine it’s to save wear (and therefore maintenance and money) on the brake pads. _How would they enforce such an ordinance, anyway?_ You hear 'em, pull 'em over, and give 'em a ticket. Imagine a purring cat crossed with a jackhammer – it’s an unmistakeable, and very loud, noise. ---
**Author:** ![Drum\_God](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drum_god/32/44_2.png) [@Drum\_God](https://boards.straightdope.com/u/Drum_God)\ **Post date:** [June 6, 2006, 2:42am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/9 "2006-06-06T02:42:24Z")
Wow, thanks for the amazingly prompt responses. Sorry I didn’t do a search first. I really thought this was more obscure than apparently it is. The only engine braking that I knew of was to shift to a lower gear or range to prevent overheating of the brakes. In our town, we have a rather long, steep grade just outside town. Our school bus drivers are recommended to put the automatic tranny into low before descending that grade. That way, the brakes have greater stopping power. I know just the noise you’re talking about. I just never knew that’s what it was. ---
**Author:** ![Airman\_Doors\_USAF](https://avatars.discourse-cdn.com/v4/letter/a/e36b37/32.png) [@Airman\_Doors\_USAF](https://boards.straightdope.com/u/Airman_Doors_USAF)\ **Post date:** [June 6, 2006, 2:50am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/10 "2006-06-06T02:50:38Z")
> [@Drum God](#): > > In our town, we have a rather long, steep grade just outside town. Our school bus drivers are recommended to put the automatic tranny into low before descending that grade. That way, the brakes have greater stopping power. That’s exactly why they use them. Out in Pittsburgh (and in other hilly areas with long descents) they have what are called [runaway truck ramps](http://upload.wikimedia.org/wikipedia/commons/f/f3/RunawayTruckRamp.jpg), which are basically gravel traps with big water barrel barriers or long runoffs, that are built for the sole purpose of catching trucks who slag their brakes going down hills. Without the compression brake that would happen with alarming frequency, I suspect. ---
**Author:** ![drachillix](https://avatars.discourse-cdn.com/v4/letter/d/48db29/32.png) [@drachillix](https://boards.straightdope.com/u/drachillix)\ **Post date:** [June 6, 2006, 2:54am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/11 "2006-06-06T02:54:42Z")
> [@Drum God](#): > > That way, the brakes have greater stopping power. The brakes don’t have greater stopping power, it just normal compresson slowing. This is pretty much the same as downshifting a manual transmission while rolling downhill. ---
**Author:** ![A.R.Cane](https://avatars.discourse-cdn.com/v4/letter/a/ee7513/32.png) [@A.R.Cane](https://boards.straightdope.com/u/A.R.Cane)\ **Post date:** [June 6, 2006, 2:58am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/12 "2006-06-06T02:58:59Z")
The term “Jake Brake” is used commonly by truckers and has been for decades. The real problem isn’t the compression brakes, it’s the muffler. Many truck owners like to modify their mufflers to reduce back pressure, unfortunately this also decreases the effectiveness of the muffler. Most jurisdictions have changed their signs to prohibit the use of “unmuffled” engine brakes. My last truck had a stock mufflers and you could barely tell when I was operating my “Jake”. Compression brakes are an important safety device. While using them on flat terrain is just a convenience and perhaps a bit of a brake saver, they are very important on downgrades. ---
**Author:** ![danceswithcats](https://avatars.discourse-cdn.com/v4/letter/d/9e8a1a/32.png) [@danceswithcats](https://boards.straightdope.com/u/danceswithcats)\ **Post date:** [June 6, 2006, 3:17am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/13 "2006-06-06T03:17:36Z")
Number one, if you’re operating a safe truck (you’ve done a proper pre-trip) and your brakes aren’t stroked out, there is no reason why the vehicle should get away from you on a downgrade, so long as you obey the signs at the beginning of the downgrade. As stated by others, JakeBrake is one name, Extarder is another. The noise can be muffled, but that reduces exhaust flow. When you’re climbing a grade with 30 trailing tons, that muffler is a power reducer. The quiet brake is an output retarder, found only on automatic transmissions. You can’t use it on downgrades without overheating the transmission, but for highway driving, I’ve used it in conjunction with Jake to darn near stop a 23 ton truck without touching foot brakes. (flat highway, anticipated deceleration) 😉 ---
**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\ **Post date:** [June 6, 2006, 3:25pm UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/14 "2006-06-06T15:25:28Z")
> [@A.R. Cane](#): > > My last truck had a stock mufflers and you could barely tell when I was operating my “Jake” Ah, so you were one of the rare operators whose Jakes sounded like a soft _Burrrrrrr_, rather than the usual _ **BRAKAKAKAKAKAKAKAKAK** _ going down the hill. Wish there were more like you along 80 between Reno and Auburn. ---
**Author:** ![A.R.Cane](https://avatars.discourse-cdn.com/v4/letter/a/ee7513/32.png) [@A.R.Cane](https://boards.straightdope.com/u/A.R.Cane)\ **Post date:** [June 6, 2006, 3:47pm UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/15 "2006-06-06T15:47:08Z")
> [@gotpasswords](#): > > Ah, so you were one of the rare operators whose Jakes sounded like a soft _Burrrrrrr_, rather than the usual _ **BRAKAKAKAKAKAKAKAKAK** _ going down the hill. > > Wish there were more like you along 80 between Reno and Auburn. Yep, and I’ve been over Donner many times, although I’ve also run down the Feather River to avoid all that chaining up nonsense. ---
**Author:** ![Drum\_God](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drum_god/32/44_2.png) [@Drum\_God](https://boards.straightdope.com/u/Drum_God)\ **Post date:** [June 7, 2006, 1:05am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/16 "2006-06-07T01:05:02Z")
> [@drachillix](#): > > The brakes don’t have greater stopping power, it just normal compresson slowing. This is pretty much the same as downshifting a manual transmission while rolling downhill. No, I understand, just wasn’t very clear. As brakes heat up, their stopping power is reduced due to their having a finite heat capacity. If entering the downgrade at a proper speed, there shouldn’t be a problem. If you hit it too fast, however, watch out. The downgrade near our town is steep and has several sharp turns in it. So how does a truck driver operated the engine brake. Does he/she flip a switch then depress the brake pedal on the floor? Does operation of the engine brake also activate the brake lights outside? ---
**Author:** ![A.R.Cane](https://avatars.discourse-cdn.com/v4/letter/a/ee7513/32.png) [@A.R.Cane](https://boards.straightdope.com/u/A.R.Cane)\ **Post date:** [June 7, 2006, 1:31am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/17 "2006-06-07T01:31:28Z")
> [@Drum God](#): > > No, I understand, just wasn’t very clear. As brakes heat up, their stopping power is reduced due to their having a finite heat capacity. If entering the downgrade at a proper speed, there shouldn’t be a problem. If you hit it too fast, however, watch out. The downgrade near our town is steep and has several sharp turns in it. > > So how does a truck driver operated the engine brake. Does he/she flip a switch then depress the brake pedal on the floor? Does operation of the engine brake also activate the brake lights outside? The vast majority of diesel engines in class 8 trucks (the typical semi tractor) are six cylinder. Most compression brakes are split, three cylinders for each circuit. The compression brake is electrically engaged by a pair of switches in the cab. If the switches are in the on position, then the brake is automaticaly engaged when the driver takes his foot completely off the accelerator. I had a foot switch in my last truck. The compression brake did not engage until I depressed the foot switch. Thre were also two toggle switches on the dash. I could select both on, one on, or both off. If both were off the foot switch, wired in series, was of no use. The reason for the split is to give the driver more control. On a lesser grade you might only need half the retardation, on a steeper grade both. It may sound complicated, but it’s very easy to operate once you become accustomed to it. It actually becomes second nature, to the point where I was trying always to hit the foot switch when driving my pickup. ---
**Author:** ![A.R.Cane](https://avatars.discourse-cdn.com/v4/letter/a/ee7513/32.png) [@A.R.Cane](https://boards.straightdope.com/u/A.R.Cane)\ **Post date:** [June 7, 2006, 1:35am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/18 "2006-06-07T01:35:21Z")
The air brakes are independant of the compression brake. I would use my right foot to depress the air brakes, while using my left to operate the compression brake, which is either on or off. ---
**Author:** ![Drum\_God](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drum_god/32/44_2.png) [@Drum\_God](https://boards.straightdope.com/u/Drum_God)\ **Post date:** [June 7, 2006, 1:38am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/19 "2006-06-07T01:38:34Z")
> [@A.R. Cane](#): > > The vast majority of diesel engines in class 8 trucks (the typical semi tractor) are six cylinder. Most compression brakes are split, three cylinders for each circuit. The compression brake is electrically engaged by a pair of switches in the cab. If the switches are in the on position, then the brake is automaticaly engaged when the driver takes his foot completely off the accelerator. I had a foot switch in my last truck. The compression brake did not engage until I depressed the foot switch. Thre were also two toggle switches on the dash. I could select both on, one on, or both off. If both were off the foot switch, wired in series, was of no use. The reason for the split is to give the driver more control. On a lesser grade you might only need half the retardation, on a steeper grade both. > It may sound complicated, but it’s very easy to operate once you become accustomed to it. It actually becomes second nature, to the point where I was trying always to hit the foot switch when driving my pickup. So you normally cruise with the switches in the off position? I take it that this is not a brake that would be used when that little Dodge Neon darts in front of you and you yell “#@#%%^!!!” when you spill your coffee and slam on the brakes. Is this a device ever used in passenger vehicles such as buses? ---
**Author:** ![Drum\_God](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drum_god/32/44_2.png) [@Drum\_God](https://boards.straightdope.com/u/Drum_God)\ **Post date:** [June 7, 2006, 1:39am UTC](https://boards.straightdope.com/t/weird-highway-sign/359566/20 "2006-06-07T01:39:36Z")
> [@A.R. Cane](#): > > The air brakes are independant of the compression brake. I would use my right foot to depress the air brakes, while using my left to operate the compression brake, which is either on or off. But isn’t your left foot busy with the clutch as you downshift? [Next page](https://boards.straightdope.com/t/weird-highway-sign/359566.md?page=2)