Low speed limits on inclines

[Zeffy]

So I've been thinking about this for awhile, and I'm curious to know what other's opinions are on it.

There is a road near me where a 25 MPH zone is situated on multiple medium-grade inclines. This area is notorious for being a speed trap, as one could imagine. The problem here is that the speed limit - which is 25 presumably because of the densely packed residential buildings - is very difficult to not exceed by more than 5 MPH without braking very hard on some of the hills that easily increase your speed by at least 10 MPH. It's avoidable, but you really have to use the brake - applying little pressure does nothing to slow you back down to 25.

The second problem is going up the hill. As we all know, you have to accelerate in order to get up a hill. One of the inclines in this area almost forces you to do at least 32 MPH to make it up the hill. If you don't accelerate, you won't make it up, simple as that. You have to put a considerable amount of pressure on the gas pedal to make it up the hill. And unless you are some car wizard, you're most likely going above 30 MPH, and Princeton cops are known to pull people over for going 3 over.

This area is one that I hate driving through, because it's difficult to maintain the speed limit when you hit an incline. Being that it's patrolled on a more frequent basis, it's a risk to go anything above 28 in this area. Some drivers ignore it, but others respect it, because you never know when a cop is lurking. I've even seen officers out on the sidewalk with a radar gun.

My solution to this problem is to bump the speed limit up by 5. I have found it much easier to maintain 30 MPH compared to 25 MPH. I don't see a lot of foot traffic either, and while there are houses everywhere, I've seen other places in New Jersey get by fine with 30 MPH in denser settings.

So, my question to everyone is this - do you feel that low speed limits on inclines that can easily cause someone to exceed that limit by more than 5 MPH should be allowed? And should they be able to be enforced strictly? I'm also curious to see where else these situations exist.

[hbelkins]

I know of a number of split speed limits for trucks on steep hills, including I-64 eastbound beyond Beckley and I-68 descending into Cumberland. I also know of a pretty bad speedtrap on the West Virginia Turnpike northbound, which has a 60 mph limit. Just about every time I have traveled that road, there's a cop waiting at the bottom of a grade just around a curve, probably waiting to nail trucks that have a hard time sticking to the limit.

[briantroutman]

Regarding the speed limit on the downhill slope, remember that you could downshift before descending the hill to keep your car's speed under control without having to touch the brakes. That's what I'd do in a known speed trap like that.

What I do wonder about, though, are what seem to be ridiculously low downhill speed limits for trucks at some locations. If I remember correctly, both downhill grades on US 15 at Bald Eagle Mountain in PA (northbound into South Williamsport and southbound toward Montgomery) are posted at 35 for trucks despite the car speed limit being 55 with prevailing speeds easily at 65 or higher–and only one lane in the downhill direction. In practice, the vast majority of truckers seem to disregard the truck speed limit and race down the mountain as fast as the cars do, but if one actually did adhere to the limit, he/she would likely pose a major collision risk to motorists going twice as fast.

[Mapmikey]

Regarding the speed limit on the downhill slope, remember that you could downshift before descending the hill to keep your car's speed under control without having to touch the brakes. That's what I'd do in a known speed trap like that.

What I do wonder about, though, are what seem to be ridiculously low downhill speed limits for trucks at some locations. If I remember correctly, both downhill grades on US 15 at Bald Eagle Mountain in PA (northbound into South Williamsport and southbound toward Montgomery) are posted at 35 for trucks despite the car speed limit being 55 with prevailing speeds easily at 65 or higher–and only one lane in the downhill direction. In practice, the vast majority of truckers seem to disregard the truck speed limit and race down the mountain as fast as the cars do, but if one actually did adhere to the limit, he/she would likely pose a major collision risk to motorists going twice as fast.

US 40 WB approaching Uniontown has a 10 mph speed limit for trucks (https://goo.gl/maps/EhFhe3DChCN2).  The car speed limit is at least 45 and it might be 55 (there are 45 mph posted curves on this descent).  But at least it is multilane all the way down...

Mike

[jeffandnicole]

It doesn't matter what the speed limit is.  Simple physics will say you will gain speed going downhill.

Now, what you are stating doesn't even make sense:

You stated that if you're on a 25 mph road: "[it's] very difficult lt to not exceed by more than 5 MPH without braking very hard on some of the hills that easily increase your speed by at least 10 MPH."

Then you state: "I have found it much easier to maintain 30 MPH compared to 25 MPH."

How is one able to maintain 30mph in a 30, but yet speed up to 35 mph in a 25?

[oscar]

US 40 WB approaching Uniontown has a 10 mph speed limit for trucks (https://goo.gl/maps/EhFhe3DChCN2).  The car speed limit is at least 45 and it might be 55 (there are 45 mph posted curves on this descent).  But at least it is multilane all the way down...

MD 135 eastbound, approaching Westernport has a 50mph cars/10mph trucks split speed limit, due to the long downhill incline ending with a sharp right turn. Trucks are required to stop twice going down the incline. There are pullouts where the trucks must stop, but otherwise the highway is only two-lane.

[oscar]

Unrelated, but on Toyota Hybrids, there is a "B" on the gear selector, standing for (I believe) "brake". It's a form of downshifting to help charge the battery.

That's an engine braking mode for the continuously variable transmission. Like on other cars, it's to help keep the brakes from overheating -- I think it's never needed to recharge the hybrid battery, which will happen quickly enough on a downhill even without B mode.

I used B mode in my Prius descending from the Pikes Peak summit. While it wasn't as good as downshifting to first gear (which I did on an earlier trip in a vehicle with a conventional four-speed automatic transmission), it was close enough.

[jakeroot]

Unrelated, but on Toyota Hybrids, there is a "B" on the gear selector, standing for (I believe) "brake". It's a form of downshifting to help charge the battery.

That's an engine braking mode for the continuously variable transmission. Like on other cars, it's to help keep the brakes from overheating -- I think it's never needed to recharge the hybrid battery, which will happen quickly enough on a downhill even without B mode.

I used B mode in my Prius descending from the Pikes Peak summit. While it wasn't as good as downshifting to first gear (which I did on an earlier trip in a vehicle with a conventional four-speed automatic transmission), it was close enough.

Indeed, it's simply a plus. I should have wrote "it's a form of downshifting to prevent brake overheating and help charge the battery (just slightly)".

[vdeane]

I don't recall learning about downshifting automatics in driver's ed either, and mine was more comprehensive than most (even including what to do if the engine stalls while driving and how the use the parking brake to stop the car if needed).  Given that I drive a standard, the idea of downshifting an automatic sounds kinda dangerous to me, as there is no clutch to disengage before shifting.

[jeffandnicole]

I don't recall learning about downshifting automatics in driver's ed either, and mine was more comprehensive than most (even including what to do if the engine stalls while driving and how the use the parking brake to stop the car if needed).  Given that I drive a standard, the idea of downshifting an automatic sounds kinda dangerous to me, as there is no clutch to disengage before shifting.

It's pretty much similar to hitting the gas to rev the engine more, or taking off the overdrive.  Doing so while driving isn't going to hurt anything. 

The main key is don't overdrive the speed for the gear.  Most 4 gear vehicles, for example, are fine at all speeds in 4th & 3rd gears; 2nd gear you probably want to stay below highway speeds, and 1st gear is used for when you're stuck and trying to get out of mud and snow. 

Really though, most car braking systems are dealing with a few thousand pounds, and should be able to handle nearly every hill without a problem.  But let's put it in perspective: NJ's peak point is only 1,800 feet high.  There are roads in the mountains that rise or fall more than that in just a few miles.  Our 'hills' that are being referred to here aren't going to be more than a few hundred feet or so in elevation at the most.  If a car can't handle breaking for that distance, there's something wrong with it.

[cpzilliacus]

MD 135 eastbound, approaching Westernport has a 50mph cars/10mph trucks split speed limit, due to the long downhill incline ending with a sharp right turn. Trucks are required to stop twice going down the incline. There are pullouts where the trucks must stop, but otherwise the highway is only two-lane.

This descent also features the only truck escape ramp in Maryland I have ever seen on an arterial highway.

GSV from 2008, but still accurate.

I believe the BYS (big yellow sign) panels seen here in 2008 have been replaced with even bigger ones.

[Zeffy]

With the exception of most of New Jersey and DelMarVa, the Northeast is anything but flat.

You should probably visit New Jersey again.


New Jersey is most definitely very mountainous, ESPECIALLY in the northern-most counties. You can see this on I-78 in Hunterdon County. South Jersey isn't the entirety of the state.

[cl94]

With the exception of most of New Jersey and DelMarVa, the Northeast is anything but flat.

You should probably visit New Jersey again.


New Jersey is most definitely very mountainous, ESPECIALLY in the northern-most counties. You can see this on I-78 in Hunterdon County. South Jersey isn't the entirety of the state.

Hence why I said "most". I know about northern New Jersey, but the majority of the land area is relatively flat

[jeffandnicole]

Probably a slight exaggeration that NJ is very mountainous, but it does have its share of hilly terrain.

[kkt]

If it's hard to keep your speed down to 25 on the descent, think how much harder it would be than usual to make a panic stop if you had to.  I haven't seen the road in question but that's probably why the speed limit is 25.  Use low gears.  You can use low gears going up, too, if you want to keep your speed at the posted limit.

I'm really appalled at the number of people who report that their drivers' ed didn't cover gearing down.

[Pete from Boston]

With the exception of most of New Jersey and DelMarVa, the Northeast is anything but flat.

You should probably visit New Jersey again.


New Jersey is most definitely very mountainous, ESPECIALLY in the northern-most counties. You can see this on I-78 in Hunterdon County. South Jersey isn't the entirety of the state.

Hence why I said "most". I know about northern New Jersey, but the majority of the land area is relatively flat

South Jersey is entirely coastal plain, and thus is indeed very flat. The extreme Northwest is part of the so-called Ridge and Valley province that is probably best known here for requiring tunnels on the Pennsylvania Turnpike.  While this is indeed mountainous for New Jersey, people in the West call these hills, not mountains.

[Pete from Boston]

If it's hard to keep your speed down to 25 on the descent, think how much harder it would be than usual to make a panic stop if you had to.  I haven't seen the road in question but that's probably why the speed limit is 25.  Use low gears.  You can use low gears going up, too, if you want to keep your speed at the posted limit.

I'm really appalled at the number of people who report that their drivers' ed didn't cover gearing down.

Driver's ed never warned me about burning out my brakes going down a hill.  This is a glaring omission that could kill somebody pretty easily.  I wasn't really fully aware of the problem until I started to burn my brakes, and they started to not work.  Fortunately it was a forgiving lesson.

[jeffandnicole]

SE NJ is flat. SW NJ has some good hills.  And good sledding hills.  Especially the ones where there's a stream at the base that you try to get as close to possible to without sliding in, and everyone knows damn well you're going to eventually slide in.

[cl94]

If it's hard to keep your speed down to 25 on the descent, think how much harder it would be than usual to make a panic stop if you had to.  I haven't seen the road in question but that's probably why the speed limit is 25.  Use low gears.  You can use low gears going up, too, if you want to keep your speed at the posted limit.

I'm really appalled at the number of people who report that their drivers' ed didn't cover gearing down.

Driver's ed never warned me about burning out my brakes going down a hill.  This is a glaring omission that could kill somebody pretty easily.  I wasn't really fully aware of the problem until I started to burn my brakes, and they started to not work.  Fortunately it was a forgiving lesson.

I never learned it in class. I'll attribute that to taking it in Buffalo, where the land is relatively flat and few people will venture often to places that require downshifting. Most people don't do much driving outside where they live and many live in the same area for their entire life, so I see why it wouldn't necessarily be taught in flat areas.

[J N Winkler]

Right... I was tired when I posted that. I meant that with average braking, I can maintain 30 MPH going down the slope. But since the speed limit is 25, 30 is too fast already, so there's no point to slow to 5 over when doing 10 over is just as bad. I have to practically slam on my brakes to maintain 25 MPH. Who wants to do that to avoid a speeding ticket?

Using the brakes to oppose a downhill grade is a bad, bad idea to begin with.

No one explained anything about gears in automatic vehicles, in both 10th grade drivers ed and the driving school I went to. And no, I don't even know how to shift gears in an automatic, nor do I want to, unless the situation really called for it.

I don't care what they thought it was necessary to teach in driver's education or at a driving school:  knowing how to downshift to control speed on inclines--whether the car is equipped with a manual or an automatic--is an essential part of safe driving.  In any case, formal instruction is only the start of learning.

As for how to downshift an automatic:  the basic thing is to choose a lower gear range.  This has the effect of prohibiting upshift to a gear lying above that range.  If you are already in such a gear, the transmission will either downshift immediately, or do so as soon as engine revs drop to such a level that the change in gear will not redline the engine.

A four-speed automatic transmission with traditional gear selector layout will allow you to choose D (or sometimes circle-D, the circle indicating lockup overdrive), where all gears are available; 3, where all gears up to 3rd are available; 2, where all gears up to 2nd are available; and sometimes 1, where only 1st gear is available.  You won't need to use 1 often (I never had to use it except on > 15% downhill grades, which are extremely rare), and many cars do not even have it in the first place.  On some cars with the traditional layout, there is no 3 and prohibition of 4th gear is accomplished by cancelling lockup overdrive (a button for this will typically be on the selector lever if it is console-mounted).

On some newer cars selection of lower gear ranges is accomplished by paddle shifters mounted to the steering wheel (one marked - for downshifting one gear with each pull, another marked + for upshifting with each pull).  On the gearshift lever in these cars there may be just D or S, the latter meaning "selective shift," i.e., the car will not shift up or down from the gear you choose with the paddle selector.  This is maximum manual control.  In these cars it is actually easier to control downhill speed since there is no need to take hands off the steering wheel to operate the gearshift lever.  My recommendation when going downhill in one of these cars is just to keep the selector lever in D (instead of moving it to S) and use the - shifter as needed to control speed without using the brakes.  Once you reach the bottom of the grade and start putting on the gas, the vast majority of these cars will simply return to their normal shifting pattern for D.

I'm sorry, is knowledge of that a requirement to drive a vehicle? Last I checked, it wasn't.

Doesn't really matter.  You can almost certainly pass a driving test for a noncommercial driver's license in a flat state without having to prove knowledge of downshifting to control speed on downgrades.  But that isn't worth a hill of beans when the brakes start smoking.

I don't recall learning about downshifting automatics in driver's ed either, and mine was more comprehensive than most (even including what to do if the engine stalls while driving and how the use the parking brake to stop the car if needed).  Given that I drive a standard, the idea of downshifting an automatic sounds kinda dangerous to me, as there is no clutch to disengage before shifting.

With automatics the key thing is to time the selection of the lower range carefully so that as little torque as practical is carried through the change in gear that almost inevitably follows.  This means reading the road ahead so you can move the lever before the car bites into the grade, and this takes some experience.  Automatics are robust enough to handle a few changes in the middle of downgrades, but this is not really consistent with good vehicle sympathy.

Automatics have clutches (and also brake bands if equipped with planetary or Ravigneaux gearsets), but these are not like the clutches in a manual.  The clutches in an automatic are basically a collection of slotted discs that fit into a clutch drum and each clutch is engaged by filling the drum with transmission fluid supplied at line pressure.  Line pressure in turn is regulated by solenoids in electronically controlled transmissions.  To allow smooth but positive engagement of a new gear, the control software sets line pressure by pulling down the raw pressure from the oil pump (which is constant-displacement and driven by the input shaft) and uses it to control the clutch fill time.  The usual objective is to allow the clutch to engage with some slip (the term of art for this is "slip modulation") so that the driver's subjective experience is one of a smooth shift, almost regardless of throttle position as reported by the throttle position sensor.  The clutch discs and the transmission fluid itself are designed to accommodate slip modulation and the former will last many hundreds of thousands of miles as long as the fluid is properly maintained.

The calculations that are used to set the clutch fill time rely on the fluid having a 100° C kinematic viscosity within a certain fairly narrow range, and over time fluid tends to shear down, so unless you are running an expensive shear-resistant full-synthetic (API Group IV/V) transmission fluid, after a while the fluid will be too thin and the transmission will be going into gear harshly.  The clutches can take only so much of this before they start slipping.

[kkt]

Agreeing with what J. N. Winkler said, but just adding that while forcing the automatic to downshift at high revs may not be ideal, if you're having trouble holding the car with just the brakes it's a hell of a lot better to put a little wear on the tranny than to boil your brake fluid.

[US 41]

I voted yes, but they should not be strictly enforced. My reasoning is that it is very hard to maintain your speed when going down a large hill, without having to get new brakes. Truck speeds on downhill inclines should be so that the trucks slow way down before going downhill, that way they aren't doing 90 mph when they get to the bottom of the hill / mountain.