Variable speed limit signs

[tradephoric]

An effective way of reducing congestion and improving safety or ineffective and wasteful?

[Zeffy]

It's a cool concept, and it's neat that they can display the FHWA fonts in a digital way, but from what I read the cost of those VMS that do allow the FHWA fonts cost a fortune. And look on that single image you posted - you've got 7 of them!

I'd rather allow the "reasonable and prudent" solution for speed limits, or when traffic isn't congested enough to warrant slow speeds, speed limits of 75+.

[1995hoo]

When they tried them on the Beltway here through the Wilson Bridge work zone, it didn't really work (we did not have lane-specific limits like those shown above, however). People ignored the variable limits. They're posting new variable limit signs on the reversible carriageway on I-95 as part of the HO/T project, coupled with lane control signals, presumably because the shoulders are being narrowed enough that they want to use variable speed limits to help manage traffic when there's an incident. I don't expect the results to be all that different. I tend to think variable speed limits COULD work if limits were ordinarily set at a number drivers consider reasonable AND if people became used to the idea that a lower limit denotes a serious reason to slow down. But I don't see that happening in most places.

[hotdogPi]

Why is the left lane slower in the picture?

[Brandon]

An effective way of reducing congestion and improving safety or ineffective and wasteful?

Nice concept, but what's the point?  There's little reason an urban freeway speed limit needs to be less than 70 mph unless there is a good, solid engineering reason for it.  Congestion is just that, congestion.

[1995hoo]

When they tried them on the Beltway here through the Wilson Bridge work zone, it didn't really work (we did not have lane-specific limits like those shown above, however). People ignored the variable limits. They're posting new variable limit signs on the reversible carriageway on I-95 as part of the HO/T project, coupled with lane control signals, presumably because the shoulders are being narrowed enough that they want to use variable speed limits to help manage traffic when there's an incident. I don't expect the results to be all that different. I tend to think variable speed limits COULD work if limits were ordinarily set at a number drivers consider reasonable AND if people became used to the idea that a lower limit denotes a serious reason to slow down. But I don't see that happening in most places.

Following up, this picture from AARoads.com is of a variable speed limit sign on US-27 near Pembroke Pines, Florida, but it's the same sort Virginia used before and will use again on the I-95 HO/T. I've passed these signs in Florida (my brother-in-law and his family live nearby) and I've always wondered what prompted their installation in that particular area.

(Edited to add: A Google search reveals it's intended to call motorists' attention to the fact that they're approaching a school zone. I've noted the school in the past because the school zone speed limit has been in effect at strange times during the summer on occasion.)

[Zeffy]

There's one in use on I-76 / the Walt Whitman Bridge heading into New Jersey:

https://www.google.com/maps/@39.903775,-75.121308,3a,19.5y,101.6h,91.33t/data=!3m4!1e1!3m2!1sWApTl7eOCFsSHOLs4Vktng!2e0

[jeffandnicole]

There's one in use on I-76 / the Walt Whitman Bridge heading into New Jersey:

https://www.google.com/maps/@39.903775,-75.121308,3a,19.5y,101.6h,91.33t/data=!3m4!1e1!3m2!1sWApTl7eOCFsSHOLs4Vktng!2e0

They are used on all 4 DRPA bridges (Walt, Ben, Barry, Betsy), with the max limit set to 45 mph.  The Del Mem Bridge uses it as well; max limit 50 mph.

[english si]

Nice concept, but what's the point?  There's little reason an urban freeway speed limit needs to be less than 70 mph unless there is a good, solid engineering reason for it.  Congestion is just that, congestion.

The whole point is to reduce it by stopping it getting exacerbated by people hitting the back end of the queue. By stopping you reaching the back end of the queue (and stopping rear ends at speed), the queue can be softened and killed. There's countless websites that show the science of this.

And the getting-on-for-two-decade trial on the M25 has been a success with VSL rolling out across the British network.



and don't forget a fun rotating prism-based sign

[tradephoric]

MoDOT Removing Variable Speed Limit Signs On I-270.  Assistant district engineer Tom Blair says the signs helped reduce slightly the number of crashes along I-270 - but the department was hoping for more.

http://news.stlpublicradio.org/post/modot-removing-variable-speed-limit-signs-i-270

[jakeroot]

Why is the left lane slower in the picture?

The point of the variable speed limits, according to WashDOT, isn't to just willy-nilly reduce the speed limit without reason. The point is to slow you down, or at least give you an idea of upcoming speeds, so you don't come over one of the many hills along I-5(N) into Seattle and fly into a stopped car at 70 mph.

In regards to the 40 mph in the HOV lane, that is not something you see everyday, but my guess is that there is either an accident/something blocking the HOV lane, or the Seneca Street ramp is backed up and the express lanes are closed.

Also, WSP doesn't enforce the variable speed limits. The whole concept seems like more of an advisory speed limit. From my experience, however, many people do slow down to the VMS speed, but still lots of people don't...that doesn't seem like an especially good idea.

EDIT:

The whole point is to reduce it by stopping it getting exacerbated by people hitting the back end of the queue. By stopping you reaching the back end of the queue (and stopping rear ends at speed), the queue can be softened and killed. There's countless websites that show the science of this.

I just repeated what English Si already said. Sorry.

MoDOT Removing Variable Speed Limit Signs On I-270.  Assistant district engineer Tom Blair says the signs helped reduce slightly the number of crashes along I-270 - but the department was hoping for more.

I guess some states have different experiences...WashDOT is installing more of them.

and don't forget a fun rotating prism-based sign

That's awesome! I don't think I've ever seen something like that.

[agentsteel53]

By stopping you reaching the back end of the queue (and stopping rear ends at speed), the queue can be softened and killed. There's countless websites that show the science of this.

correct.  there's even research in adaptive cruise control that keeps vehicles sufficiently spread out to prevent jams.

my rule is "never hit the brakes unless I absolutely have to".  I will use engine braking, or plan ahead and not speed up as much as I otherwise would have - everything to prevent those dreaded red lights in the back. 

[1995hoo]

....

and don't forget a fun rotating prism-based sign

I just got this mental image of the back side of that sign reading "20" with a British version of Rosco P. Coltrane sitting down the road flipping the number around when he wants to give someone a ticket.

[tradephoric]

Voluntary compliance severely undermines the effectiveness of variable speed limits.  Drivers have little incentive to reduce their speed if they know it's not enforceable.  The average driver doesn't care about the theory behind variable speed limits.  All they are thinking is.... "these traffic engineers are idiots... telling me to slow down for no reason again!". 

[english si]

That's awesome! I don't think I've ever seen something like that.

I haven't either (for a speed limit), but it was in SABRE's VSL gallery and thought it was cool. Rotating Prisms have been around for years and are relatively common (even if they don't rotate often).

I just got this mental image of the back side of that sign reading "20" with a British version of Rosco P. Coltrane sitting down the road flipping the number around when he wants to give someone a ticket.

Voluntary compliance severely undermines the effectiveness of variable speed limits.

Note that the electronic signs have a speed camera sign on the sign bridge - this allows compliance. But note that the camera is on the sign bridge itself and there's a grace period in case the sign changes as you pass under it. Certainly a Rosco P. Coltrane will have real issues getting the ticket approved.

[tradephoric]

And the getting-on-for-two-decade trial on the M25 has been a success with VSL rolling out across the British network.

Any ideas why they have been more popular and successful in Britain?  According to assistant district engineer Tom Blair, the variable speed limit signs along I-270 in Missouri saw only a slight reduction in total crashes.  Apparently the reduction in crashes wasn't large enough to justify keeping the variable signs in place and they have since been yanked out.   What kind of crash reduction has Britain seen with the M25 trial?

[DrSmith]

They have done this for as long as I remember (and probably much longer) on the Jersey Turnpike, stating there is congestion and then showing and speed limit of 45, occasionally 35 if it is just plain packed with stop and go.  However, it seems ineffective because no one follows the reduced speed, of course the 65 mph speed limit is routinely ignored with some parts flowing at 90. 

The other problem with this approach (at least as I have seen it on the Turnpike) is there they do not pay attention and update frequently and quickly. There are times where many miles of highway still say congestion and 45 mph speed limit when the delays have completely cleared and traffic is flowing better.  Maybe they are better now, but I think it trains people to pay less attention and makes it more ineffective.  Personally, I do like the way CT handles the messaging.  There are no variable speed limits, but the VMS have consistent simple messaging on 3 lines, DELAY; 6 MILES; EXITS 37-32 (or at least that is one message I frequently get to enjoy on the morning drive on 91 southbound). The other advantages I think is they make a significant effort to keep the information up to date so you know what to expect and change them frequently with the locations of the delays, accidents, road work, etc. Also, they don't tend to post a bunch of other messages too much, and leave them off when there isn't something related to road conditions to post for the most part.  I would believe this helps people to pay attention to what is posted and know of actual road conditions and to prepare rather than ignoring it because it always says something.

[jakeroot]

Even if they aren't very effective at slowing traffic (which is variable depending on the jurisdiction), they do have the ability to indicate things other than speed limits:



This is a render but it's an accurate representation of reality.

Of course, sometimes it's not always a good day:

[agentsteel53]

that is critical.  telling you which lane is closed. 

[tradephoric]

Note that the electronic signs have a speed camera sign on the sign bridge - this allows compliance. But note that the camera is on the sign bridge itself and there's a grace period in case the sign changes as you pass under it. Certainly a Rosco P. Coltrane will have real issues getting the ticket approved.

Rosco will have a hard time getting the speed camera tickets approved since they haven't been working for a year.

M25: not a single motorist caught by speed cameras in a year
http://www.telegraph.co.uk/motoring/news/9740880/M25-not-a-single-motorist-caught-by-speed-cameras-in-a-year.html

[ski-man]

There are these on I-70 in Colorado and I-80 in Wyoming, and they are used to regulate traffic due to snowy conditions.

[SteveG1988]

Why is the left lane slower in the picture?

Looks like a left exit up ahead

Edit:

On second thought, it looks more like a HOV lane entry as well

[jeffandnicole]

According to assistant district engineer Tom Blair, the variable speed limit signs along I-270 in Missouri saw only a slight reduction in total crashes.  Apparently the reduction in crashes wasn't large enough to justify keeping the variable signs in place and they have since been yanked out.

Even if one life is saved...ah, never mind.

Numerous things have been used that turn it to be less than effective but they remain. Wonder what was different about these vsls.

[US71]

Texas has a few. I saw one near Linden near a school, so I'm assuming the speed limit changes when school is open.

[SidS1045]

New Hampshire has a few of these in the construction zone of I-93 between Derry and the Massachusetts state line.