Traffic lights on an interstate

[1995hoo]

Technically it's a roundabout.

what is the difference?

I know there is a difference, but I can never keep track.  I think in some states it is "roundabout: traffic already inside has right of way, while traffic circle: traffic entering has right of way" while in other states it is the other way around, and then there is a thing called a rotary which really mucks things up.

The primary difference has to do with the design and the speeds of traffic. "Traffic circles" generally have approaches that run tangent to the circle and are designed for entry and exit at full speed. Traffic entering the circle often has to merge (think of how the loop-around ramps on a cloverleaf require a weave area–a traffic circle's tangent approaches work similarly in terms of entering and exiting traffic having to cross paths). In a multi-lane traffic circle, entering traffic may have to yield only to traffic in the outer lane (although I know of at least one circle where traffic already on the circle has to yield to any entering traffic), whereas on a roundabout entering traffic has to yield to everyone. Generally to exit a traffic circle you have to change lanes to the outer lane, whereas on a roundabout you can usually exit from any lane. Roundabouts are generally smaller than traffic circles and are designed to be navigated at slower speeds.

A large portion of the general public doesn't know that there's a distinction between a traffic circle and a roundabout, but the difference is real. If you've driven in the District of Columbia and encountered their circles, you'll understand why a lot of people in the DC area have an irrational fear of any circular road junction. DC has traffic circles, not roundabouts. They don't work the same way (one might question whether DC's work at all).

I suppose you could validly say that a roundabout is a particular type of traffic circle with specific design characteristics, but that's mighty cumbersome to say.

[agentsteel53]

ah okay, so the definition has little to do with the access rules.  I tend to think that I generally have to yield to traffic already in the circle; unless I see signs explicitly contradicting my ability to do so, I would take right of way over incoming traffic if I were already inside. 

at least in the US, where absolutely no one has any idea what "priority to the right" means.  Go ahead and try driving "priority to the right" in a shopping mall parking lot, or similar place with no explicit traffic controls.  The general rule around here tends to be "traffic making a turn yields to traffic going straight" - and in the case of two drivers intending to go straight, then road of seemingly "larger" size is given priority.  since this is all happening at 5mph, irateness is the worst consequence.

also: what is a rotary, then?  the Massachusetts-preferred term for a traffic circle?  see: Cape Cod Rotary.

[vtk]

^ Operationally, a roundabout is actually a collection of four (or fewer or more) yield-controlled intersections between one-way roads which may or may not have multiple lanes.  A traffic circle is like a circular freeway that only travels counter-clockwise.  A traffic circle has a ring road with one or more continuous "through" lanes that go around the circle.  A roundabout would require drivers to "turn left" repeatedly to stay on an imagined ring road.

And it's my understanding that "rotary" is a New England / Canada synonym for "traffic circle".

[Duke87]

In 1994 or so we went on a trip to the Liberty Science Center and I was able to convince my father to drive home through the Holland Tunnel rather than go over the GW Bridge (he subsequently swore never again - but this was before the West Side Highway was redone and streamlined). That would have been my first experience with traffic lights on an interstate, although I don't remember noting that at the time. I do remember that in order to avoid paying an extra toll, we used local streets to get to the tunnel rather than take the Turnpike from 14B to 14C - probably why I didn't notice anything odd.

I can also say I've done 676/Ben Franklin Bridge.

If we want tunnels and drawbridges:
- Fort Pitt and Squirrel Hill Tunnels in Pittsburgh
- Wheeling tunnel on I-70
- all four tunnels on the mainline PA Turnpike
- (old) Woodrow Wilson Bridge in DC
- all four tunnels in New York City (although only 2 of them are technically on an interstate...)
- Bronx River drawbridge on I-278
- Hutchinson River drawbridge on I-95 (replaced with a non-moveable span in 1995, but I was on it before then many times!)

[hbelkins]

Hey, how about I70 in Pennsylvania. That has traffic lights, too.

That doesn't really count, since technically that's US 30.

[Brandon]

Hey, how about I70 in Pennsylvania. That has traffic lights, too.

That doesn't really count, since technically that's US 30.

However, last time I was through there (1991), it was signed as I-70 as well as US-30.

[Ian]

Hey, how about I70 in Pennsylvania. That has traffic lights, too.

That doesn't really count, since technically that's US 30.

Then neither would I-676, as PennDOT only considers the surface level section of road between Vine Street and the bridge as only US 30.

[NE2]

Then neither would I-676, as PennDOT only considers the surface level section of road between Vine Street and the bridge as only US 30.

Well, yes and no. They certainly post signs pointing I-70 and I-676 through traffic lights without 'to' plates. What they don't do is internally call them 0070 and 0676. But there are other differences between state route and traffic route numbers, such as PA 380 and the east end of PA 291.

[roadfro]

Technically it's a roundabout.

what is the difference?

I know there is a difference, but I can never keep track.  I think in some states it is "roundabout: traffic already inside has right of way, while traffic circle: traffic entering has right of way" while in other states it is the other way around, and then there is a thing called a rotary which really mucks things up.

The primary difference has to do with the design and the speeds of traffic. "Traffic circles" generally have approaches that run tangent to the circle and are designed for entry and exit at full speed. Traffic entering the circle often has to merge (think of how the loop-around ramps on a cloverleaf require a weave area–a traffic circle's tangent approaches work similarly in terms of entering and exiting traffic having to cross paths). In a multi-lane traffic circle, entering traffic may have to yield only to traffic in the outer lane (although I know of at least one circle where traffic already on the circle has to yield to any entering traffic), whereas on a roundabout entering traffic has to yield to everyone. Generally to exit a traffic circle you have to change lanes to the outer lane, whereas on a roundabout you can usually exit from any lane. Roundabouts are generally smaller than traffic circles and are designed to be navigated at slower speeds.

A large portion of the general public doesn't know that there's a distinction between a traffic circle and a roundabout, but the difference is real. If you've driven in the District of Columbia and encountered their circles, you'll understand why a lot of people in the DC area have an irrational fear of any circular road junction. DC has traffic circles, not roundabouts. They don't work the same way (one might question whether DC's work at all).

I suppose you could validly say that a roundabout is a particular type of traffic circle with specific design characteristics, but that's mighty cumbersome to say.

A roundabout is designed for slower speeds on the approach with entering traffic always yielding to circulating traffic (always a yield control). You are not supposed to change lanes within the roundabout, so for properly-designed multi-lane roundabouts this means positioning yourself in the proper lane for your intended travel path prior to entering the roundabout and following the pavement markings to guide you towards the outside for your exit--you cannot always exit from any lane (it depends on the design--see 2009 MUTCD chapter 3 for examples). Pedestrians are not allowed on the central island.

A traffic circle is typically designed for slightly higher speeds, although the approaches are not always tangent to the circle to make this happen. Depending on the design, entering traffic may have to yield to circulating traffic, or it could be the other way around...some traffic circles are even stop sign or signal controlled (such as many in DC area). A traffic circle is generally larger in radius, such that sometimes there are public features (i.e. park) within the circle that pedestrians can cross to.


1995hoo is correct in that, like the saying from geometry that "a square is a rectangle, but a rectangle is not a square", a roundabout is a traffic circle but a traffic circle is not a roundabout.

DC's traffic circles, most of which (at least the ones I saw) are traffic signal controlled, are an absolute nightmare...

[PAHighways]

Then neither would I-676, as PennDOT only considers the surface level section of road between Vine Street and the bridge as only US 30.

Well, yes and no. They certainly post signs pointing I-70 and I-676 through traffic lights without 'to' plates. What they don't do is internally call them 0070 and 0676. But there are other differences between state route and traffic route numbers, such as PA 380 and the east end of PA 291.

More often than not, the lowest number of the highest classification is what is used as the SR for a multiplexed alignment.

In the case of 676, PennDOT acknowledge it as being between 76 and 95, which contradicts what DRPA and NJDOT believe.  In order to avoid driver confusion, it exits itself and is signed through Franklin Square as well.

[froggie]

DC has traffic circles, not roundabouts.

For the most part, yes.  But DC has at least one bona-fide roundabout now:  on Brentwood Rd NE south of Rhode Island Ave.

[ethanman62187]

don't forget border patrol and agricultural (also border patrol) inspection stations.

That does not count as a traffic light on an interstate. The same thin goes for toll booths.

[ethanman62187]

- (old) Woodrow Wilson Bridge in DC

It's still signalized, but it just results in fewer openings.

[1995hoo]

DC has traffic circles, not roundabouts.

For the most part, yes.  But DC has at least one bona-fide roundabout now:  on Brentwood Rd NE south of Rhode Island Ave.

Indeed you are correct. I totally forgot about that one because I never drive in that area.

[kkt]

Some interstates have traffic lights. When did you first cut through an interstate highway that has traffic lights?  Where is it and what interstate number?

Not sure if this counts, but the toll crossing on the S.F-Oakland Bay Bridge (I-80).  As a passenger, I wouldn't have been more than a few months old the first time.

I-5 Interstate Bridge crossing from Washington to Oregon, I had to stop for the bridge, amazed the whole time that a high-level bridge hadn't been built yet.  That was about 1982...

[deathtopumpkins]

Since there are really only 4 examples in the country (676, 70, 78, and 180) I can easily tell I've only been through one (70), but if you throw in drawbridges/tunnels (and not just ones with lane-use signals), the list jumps quite a bit, and now includes:
[only interstates]
- I-64 Hampton Roads Bridge-Tunnel (which I have been stopped at more times than I can count)
- I-664 Monitor-Merrimac Memorial Bridge-Tunnel (which I have been stopped at)
- I-264 Downtown Tunnel
- I-264 Berkeley Bridge
- I-64 High Rise Bridge
- I-95 Fort McHenry Tunnel
- I-895 Baltimore Harbor Tunnel
- I-95/495 Woodrow Wilson bridge
and a whole bunch more in the northeast that I can't think of off the top of my head.

[ethanman62187]

Some interstates have traffic lights. When did you first cut through an interstate highway that has traffic lights?  Where is it and what interstate number?

Not sure if this counts, but the toll crossing on the S.F-Oakland Bay Bridge (I-80).  As a
passenger, I wouldn't have been more than a few months old the first time.

I-5 Interstate Bridge crossing from Washington to Oregon, I had to stop for the bridge, amazed the whole time that a high-level bridge hadn't been built yet.  That was about 1982...

Hey, tolls don't count towards it.

[NE2]

But drawbridges do? Who are you to define this?

[Brandon]

But drawbridges do? Who are you to define this?

Why should tolls count?  If done right in the modern era, they should be non-stop ORT, not a large plaza anymore.  So, maybe the old-style toll roads count, but modern and properly updated ones don't.

[J N Winkler]

Also: what is a rotary, then?  the Massachusetts-preferred term for a traffic circle?  see: Cape Cod Rotary.

Rotary was actually the standard AASHO term for a circular intersection designed for entry at relatively high speed.  It was used in, e.g., the 1965 Blue Book, which deprecated rotaries in general but laid down design standards for them in accord with contemporary American engineering practice, which at that time was not cognizant of British research into the benefits of a yield-on-entry rule.  After the benefits of a yield-on-entry rule were shown, it took about a decade and a half of additional research to elucidate the ways in which operation of roundabouts could be improved by manipulating various geometric design parameters.  In Massachusetts state law requires yield on entry at rotaries regardless of date of design or the geometric standards used, so an old-school Massachusetts rotary with large central island works similarly to a large British roundabout built around 1965 (i.e., after introduction of yield-on-entry but before detailed research into the effects of manipulating geometric design parameters).

Recently built roundabouts, which are often called "modern roundabouts" to distinguish them from traffic circles, rotaries, and other types of circular intersection, are the culmination of decades of research not just into yield on entry but also into the effects of various geometric design parameters.  Modern roundabouts require yield on entry, but you cannot have a modern roundabout just by imposing a yield-on-entry rule.  Thus, the Massachusetts rotaries operate like modern roundabouts but are not themselves modern roundabouts and so handle traffic somewhat less efficiently.

[rickmastfan67]

I-70 Breezewood, PA
I-381 Bristol, VA (Yes, it ends at a traffic light)

Not counting Interstates that have tunnels/draw bridges, otherwise I could add several more.

[akotchi]

I have been on three of the four previously referenced:  I-70 (Breezwood), I-676 (Philly) and I-78 (Holland Tunnel)

[bassoon1986]

well if we are counting interstates that end at traffic lights, then does I-49 in Shreveport count? or is that a stub because they'll continue it northward (someday...)

[Ian]

I-381 Bristol, VA (Yes, it ends at a traffic light)

If we are counting interstates that end at signals, then I've been to...
I-587 in Kingston, NY
I-395 in Baltimore, MD
I-83 in Baltimore, MD
I-395 in Washington, DC
I-78 in New York

[rickmastfan67]

well if we are counting interstates that end at traffic lights, then does I-49 in Shreveport count? or is that a stub because they'll continue it northward (someday...)

That would be a stub since officially, I-49 still ends @ I-20.