Traffic Technicalities Times Three

August 22, 2006 7:59 AM

A flurry of interrelated USA speed limit questions:

At what point does the speed limit (USA) become so, at the point of the sign, when you see it, or when you gradually acc/decelerate that far?

Relatedly, do the exit/offramp yellow-sign speed limits actually count? How do they expect me to go from a freeway 60 to an offramp 30 and back to an access road 50? Is this implausible 30 just a recommendation, or can I actually get ticketed for doing 60 in a 30 on an offramp?

Smart-a$$ily speaking, I have never seen a speed limit on an on-ramp -- does this mean I can go 220 until I see a speed limit sign?

posted by vanoakenfold to travel & transportation (22 comments total)

I learned at driving school (in AZ) that it goes into effect as soon as the sign is visible.
posted by hermitosis at 8:07 AM on August 22, 2006

OK, do they actually count? I'm pretty sure they're just as binding as any other street sign, but I doubt anyone's pulling you over on the actual offramp.

OK number two, I'm pretty sure the 30 is lower than what you could actually achieve, but considering inclement weather and poor tires it's a lot safer to put 30. I am pretty sure they aren't just "guessing" but then again CT thinks its safer to have 50mph interstates and I don't see any engineers objecting. If i remember my physics class correctly most cars really shouldn't go much higher than 45 around those things or you risk fiery death.

OkIII, only if you drive a nice car dawg
posted by shownomercy at 8:10 AM on August 22, 2006

yes, off ramp speed limit signs count ... in michigan, the speed limit begins at the actual sign
posted by pyramid termite at 8:12 AM on August 22, 2006

It's different in every state. This page has links to most of the state laws.

Relatedly, do the exit/offramp yellow-sign speed limits actually count?

In CT, they are not legally enforceable, "except under the basic speed law, which states that motorists must operate at speeds that are reasonable and prudent for conditions."

does this mean I can go 220 until I see a speed limit sign?

No, there is a statutory speed limit that applies when there are no signs posted.

The wikipedia has a huge page on speed limits.
posted by smackfu at 8:12 AM on August 22, 2006

In Illinois we were taught the speed limit goes into effect where the sign is (as soon as you cross that imaginary line). Also, yellow signs are recommended, white or red signs (and maybe black?) are mandatory, the other colors are pretty obvious (green for miles, blue for information/services, orange for construction)
posted by chndrcks at 8:12 AM on August 22, 2006

At what point does the speed limit (USA) become so, at the point of the sign, when you see it, or when you gradually acc/decelerate that far?

My understanding has always been that it's at the point of the sign. Basically, you have from the time you see the sign to the time you pass the sign to adjust your speed accordingly. That's why you often see signs like SPEED LIMIT 35 AHEAD on curvy roads where the visual lead time wouldn't be sufficient.

How do they expect me to go from a freeway 60 to an offramp 30 and back to an access road 50?

You seem to be treating the speed limit as a recommended speed, which it absolutely is not. It's the maximum safe speed.

Is this implausible 30 just a recommendation, or can I actually get ticketed for doing 60 in a 30 on an offramp?

Yes, you absolutely can. I have.

I have never seen a speed limit on an on-ramp -- does this mean I can go 220 until I see a speed limit sign?

No. Every U.S. state has its own statewide speed limit, which applies on any unmarked road.
posted by jjg at 8:18 AM on August 22, 2006

Speed tickets are more discretionary than you would imagine. I got a ticket going 20MPH in a 45MPH because it was too fast for the conditions.
posted by geoff. at 8:28 AM on August 22, 2006

All yellow diamond signs denote "caution" and yellow speed limit signs may be telling you the maximum suggested safe speed in bad weather - that's the case in California, Washington, and Colorado anyway.
posted by dbmcd at 8:30 AM on August 22, 2006

does this mean I can go 220 until I see a speed limit sign?

no - in addition to statutory default speed limits, pretty much every state has some variation on the basic speed law which means you can be ticketed for driving fast enough to be unsafe, even if it is less than the posted speed limit.

police officers are granted considerable freedom of judgement to determine that you are driving too fast - for instance, barreling along at 65 in heavy fog is bound to get you a ticket. an ex of mine got one of these after driving off the road in heavy snow once.
posted by sergeant sandwich at 8:32 AM on August 22, 2006

I've rarely found it difficult to safely deaccelerate to an offramp 30 then hit an onramp 50.
posted by KirkJobSluder at 8:42 AM on August 22, 2006

Oh, and those guidelines are also more critical for trucks which are at higher risk for tipping on tight curves.
posted by KirkJobSluder at 8:44 AM on August 22, 2006

White signs are "regulatory" and yellow signs are "warning". This is federally mandated, state and local jurisdictions can't make interpretations on this. So any speed limit sign in yellow serves only as a caution and is not a mandatory speed.

The location of the speed sign is sometimes the starting point of the speed change but not always. Usually the change happens at a specific intersection and is specified in the municipal code. There should be some leeway on this and if a cop busts you for speeding while you are decelerating before you get to the sign, you generally can get the citation removed in court.

jjg, the speed limit is rarely the maximum safe speed. Designers typically design a road for 5-10mph over the speed limit.
posted by JJ86 at 8:52 AM on August 22, 2006

White signs are "regulatory" and yellow signs are "warning". - JJ86

This also applies in Canada. However, the same rider about how cops can ticket your for going too fast for road conditions (even if it's less than the posted limit) also applies.

Remember it's called a speed limit, not The Speed You Must Drive On This Road.
posted by raedyn at 8:58 AM on August 22, 2006

This all depends, but in Ontario (not the USA, but hell, similar enough) there are limits on ALL roads, whether marked or not. AFAIK, unless the city posts a sign at it's limits stating something less (A local example is Guelph) then the HTA (again, for Ontario) specifies the maximum speeds for different types of roads.

Those are the maximums, period, signed or unsigned. The signs can either be informative (showing the same information as the legislated maximums) or they can lower the speed (and yes, you have to obey the new lowered minimum). Now, signs that don't say "MAXIMUM" or "SPEED LIMIT" on them don't count as anything but warning signs (again, here). You really should check the legislation in the state you are windering about.

So, for an on-ramp, check the legislation. I know they're likely not specifically in there, in which case, they'll probably fall under the slowest speed (40 km/h, here, I think).

Help? Probably not. Sorry.
posted by shepd at 9:55 AM on August 22, 2006

I have a civil engineer brother-in-law who works for UDOT (Utah). I asked him about speed tolerances in road design, and he said that they DO NOT design roads to handle speeds over the planned speed limit, because that would cost money, and they don't see any compelling reason to do so.

I'm not saying it's great logic (or that it isn't), I'm just saying that's what UDOT does.
posted by SlyBevel at 9:59 AM on August 22, 2006

SlyBevel mentioned: I have a civil engineer brother-in-law who works for UDOT (Utah). I asked him about speed tolerances in road design, and he said that they DO NOT design roads to handle speeds over the planned speed limit, because that would cost money, and they don't see any compelling reason to do so.

I'm not saying it's great logic (or that it isn't), I'm just saying that's what UDOT does.

The only time when you really need to do really worry about designing for speeds is on curves, both horizontal and vertical. And anytime you engineer something you have to design in a factor of safety. It's a way to cover your ass and your license. There are also emergency vehicles that may need to use that highway at higher speeds than marked which you have to account for.

Designing an extra factor of safety in a horizontal curve for 5 mph doesn't really amount to much of a difference. Cost-wise, there is no real difference. You just bump up the superelevation. On a wide open interstate, that is simple to design and construct. In a tighter intechange it may require some fitting but again, I can hardly see where cost would factor in. On straight roads speed in design is not an issue.
Especially on interstate highways, the design engineer still needs to follow FHWA specs.
posted by JJ86 at 10:51 AM on August 22, 2006

In reference to design tolerances, it seems like people are talking past each other... but saying the same essential thing: A road with a speed limit of 55 mph is usually designed for precisely 55 miles per hour... with the required safety margins added on.

So, given that visibility, traffic load, potential inclement conditions, and a relatively low default assumption of vehicle capability (think heavy truck, or undersprung '80s minivan) all factor into the safety margin... a modern and capable personal car might well be able to safely travel this '55 mph' road in excess of 70 mph (in clear conditions). This is probably what people recognize as the 'speed tolerance'.

As was stated above, in the US/Canada, white signs are the actual speed limits (do not exceed under penalty of citation), with default (never exceeded) regional maximum speeds for particular types of roads. Yellow signs are cautionary, and are usually laid where the roads design speed has decreased suddenly (and temporarily) for some reason; blind corner, rise or turnout, tight or decreasing radius curve, etc.

You would generally be wise to obey the sprit of those cautionary signs... and if you're a good driver, these are usually places you'd likely be slowing down even without extra signage. But if the white sign says '55', the yellow sign says '30', and you go '45'... you almost certainly won't get speeding ticket ('speeding, 15 mph over limit') but you might get a 'unsafe speed for conditions', or 'basic speed law' violation... which often cost more (dollars, and license points, where applicable).
posted by zeypher at 11:46 AM on August 22, 2006

Emphasizing zeypher and others, literally every state has a statute describing your requirement to drive at speeds safe for conditions. Every state gives itself quite a bit of leeway to cite you for exceeding safe speeds, no matter what is posted.
posted by Mr. Blint at 12:18 PM on August 22, 2006

zeypher said: In reference to design tolerances, it seems like people are talking past each other... but saying the same essential thing: A road with a speed limit of 55 mph is usually designed for precisely 55 miles per hour... with the required safety margins added on.

You are exactly.....wrong. The design speed includes the FS, it isn't added on later. So for example, the posted speed is 55mph, then the design speed may be 60mph or more. It can be the same, yes but not always. Read AASHTO's Policy on Geometric Design of Highways and Streets, Chapter 2, Design Controls and Criteria.
posted by JJ86 at 12:40 PM on August 22, 2006

A road with a speed limit of 55 mph is usually designed for precisely 55 miles per hour...

Not even close. Big-I Interstate highways were originally designed, in general, to be signed at 70 or 75mph, which is, as is customary for speed-signage, the 85th percentile speed of the actual engineering surveyed traffic observed on the road. (My dad did this stuff for a living for 20 years Up North.)

You can assume that the maximum safe designed speed for I-10 is about 95mph, pretty safely (no pun intended, really).

In addition to JJ86's reference, signage geeks might also enjoy The Manual On Uniform Traffic Control Devices, which is incorporated in many state laws by reference.
posted by baylink at 1:12 PM on August 22, 2006

I know in Texas you can get a speeding ticket on an off-ramp. A friend's boyfriend had this happen several years ago. I don't know the exact circumstances, because I only heard the anecdote later, but it did happen.

Also, in Maryland I believe that the speed limit goes into effect at the moment you pass the sign.
posted by Alterscape at 2:37 PM on August 22, 2006

The problem I have with "when you see it" speed limit effective ranges is that not everyone can see it clearly at the same distances. This would also apply to offramp speed limits, because the offramp limits are usually in much much larger numbers than typical speed limit signs and can be seen much further away (as it were) while still on the freeway, so the freeway speed limit couldn't possibly be 30mph from so far back.

In cases of roads not being safe enough for above-speed travel, I would question that on the basis of raised limits in the future -- I believe Oklahoma's highway limits have been raised at least twice since I've been alive, now at 70mph.
posted by vanoakenfold at 7:45 AM on August 23, 2006