The Bowtie Intersection is finally getting its moment in the sun

[kernals12]

At some point in the near future, the people of Prince William County, Virginia will bear witness to a great revolution in traffic engineering. The VDOT plans to turn the current intersection between the Prince William Parkway and Clover Hill Road into a Bowtie Intersection and it is a thing of beauty


2 roundabouts will be installed on Clover Hill on opposite sides of the Parkway. All Left Turns at the main intersection will be prohibited and instead drivers will use the roundabouts to make a u turn to complete their movements. It's essentially an at-grade version of a dogbone interchange. It means the main intersection can use a simple 2 phase signal. It improves traffic capacity and safety.

There are many so-called innovative intersections that do the same thing such as the RCUT, the Michigan Left, the Jughandle, and the Continuous Flow Intersection, but those require a massive amount of right-of-way. This does not, in fact, as you can see, by getting rid of the need for left turn lanes, it takes up less space than a conventional intersection. I hope that this is the beginning of a glorious revolution.

[hotdogPi]

How does a median U-turn require so much ROW? There already appears to be a wide enough median.

[kernals12]

How does a median U-turn require so much ROW? There already appears to be a wide enough median.

Maybe not this particular example, but many arterials that could benefit from eliminating left turns don't have enough space for a Michigan left that can accommodate big trucks and buses, and almost all of those require signalization. Also, by leaving the median in place, it allows space for future widenings.

[1995hoo]

.... I hope that this is the beginning of a glorious revolution.

That would be more likely if this project were in the Williamsburg area.... 

How does a median U-turn require so much ROW? There already appears to be a wide enough median.

This is a Street View of that intersection now, BTW.

https://goo.gl/maps/ttgKcFdH3Fp2JBFo7

[thspfc]

Anyone care to explain the benefits of this versus a regular roundabout or even traffic light? It has to be more expensive, and there's no way it decreases travel time with all of that extra driving.

[kernals12]

The other good thing about it is it should be relatively simple to convert into a dog bone interchange. You'd just need grade separate the main intersection and then build entrance and exit ramps to those roundabouts. And turning VA 234 into a freeway between I-66 and Brentsville Road is in the Long Term Plans.

I think that it would be practical to build these types of intersections on MA 9 and especially MA 2.

[hotdogPi]

Anyone care to explain the benefits of this versus a regular roundabout or even traffic light? It has to be more expensive, and there's no way it decreases travel time with all of that extra driving.

It's a 2-phase signal. If there are left turn lanes, it's a 4-phase signal.

[kernals12]

Anyone care to explain the benefits of this versus a regular roundabout or even traffic light? It has to be more expensive, and there's no way it decreases travel time with all of that extra driving.

It gets rid of left turn phases at the main intersection so more green time can be given to people going straight. And while people making left turns will have to drive further, they will spend less time waiting at red lights.

And roundabouts don't work when traffic going in one direction is much greater than traffic going in another.

[thspfc]

Anyone care to explain the benefits of this versus a regular roundabout or even traffic light? It has to be more expensive, and there's no way it decreases travel time with all of that extra driving.

It gets rid of left turn phases at the main intersection so more green time can be given to people going straight. And while people making left turns will have to drive further, they will spend less time waiting at red lights.

And roundabouts don't work when traffic going in one direction is much greater than traffic going in another.

So does that decrease travel time overall then? I suppose it depends on how long those light phases were.

[kernals12]

Anyone care to explain the benefits of this versus a regular roundabout or even traffic light? It has to be more expensive, and there's no way it decreases travel time with all of that extra driving.

It gets rid of left turn phases at the main intersection so more green time can be given to people going straight. And while people making left turns will have to drive further, they will spend less time waiting at red lights.

And roundabouts don't work when traffic going in one direction is much greater than traffic going in another.

So does that decrease travel time overall then? I suppose it depends on how long those light phases were.

I'm guessing the number of people going straight on the Prince William Parkway outnumber all other traffic movements at that intersection, so on average, travel times would decrease. And I think the people who are going left will find it feels faster since they aren't waiting at traffic lights.

[kphoger]

How does a median U-turn require so much ROW? There already appears to be a wide enough median.

This is a Street View of that intersection now, BTW.

https://goo.gl/maps/ttgKcFdH3Fp2JBFo7

I also want to point out that, in this particular instance, the roundabouts were specifically designed with airport truck traffic in mind.  Trucks are less able to navigate median U-turns unless the median is of sufficient width.

[thspfc]

Anyone care to explain the benefits of this versus a regular roundabout or even traffic light? It has to be more expensive, and there's no way it decreases travel time with all of that extra driving.

It gets rid of left turn phases at the main intersection so more green time can be given to people going straight. And while people making left turns will have to drive further, they will spend less time waiting at red lights.

And roundabouts don't work when traffic going in one direction is much greater than traffic going in another.

So does that decrease travel time overall then? I suppose it depends on how long those light phases were.

I'm guessing the number of people going straight on the Prince William Parkway outnumber all other traffic movements at that intersection, so on average, travel times would decrease. And I think the people who are going left will find it feels faster since they aren't waiting at traffic lights.

That makes sense.

[jeffandnicole]

It will cut down on travel time...if used properly. 

These aren't conventional roundabouts.  Traffic going straight will probably fly thru the roundabout.  The traffic within the roundabout may have a tendency to yield.  If you have two cars waiting, then you start jamming up the roundabout.

Why even have a roundabout anyway?  What are the chances of a car making a u-turn on Clover approaching Prince William?  If a motorist on Clover really needed to make a u-turn, they can just do so after going thru the intersection.

[kphoger]

Why even have a roundabout anyway?  What are the chances of a car making a u-turn on Clover approaching Prince William?  If a motorist on Clover really needed to make a u-turn, they can just do so after going thru the intersection.

I agree that dual teardrops would be better.  On the other hand, if nobody will be making those movements, then it really doesn't hurt anything to have the pavement available there, plus it gives snow plows an extra option.

[hotdogPi]

I've thought about something similar here in Lowell. It's currently a 4-phase signal. There's a rotary interchange to the east, and there's an authorized vehicles only median U-turn to the west that could become public usage. This would make it a 2-phase signal if left turns are banned.

[jakeroot]

Anyone care to explain the benefits of this versus a regular roundabout or even traffic light? It has to be more expensive, and there's no way it decreases travel time with all of that extra driving.

It's a 2-phase signal. If there are left turn lanes, it's a 4-phase signal.

The addition of left turn lanes has no bearing on the number of phases. Many large intersections have no protected turn phase.

[kernals12]

I've thought about something similar here in Lowell. It's currently a 4-phase signal. There's a rotary interchange to the east, and there's an authorized vehicles only median U-turn to the west that could become public usage. This would make it a 2-phase signal if left turns are banned.

That median U turn is way too far from the main intersection.

[BrianP]

I've thought about something similar here in Lowell. It's currently a 4-phase signal. There's a rotary interchange to the east, and there's an authorized vehicles only median U-turn to the west that could become public usage. This would make it a 2-phase signal if left turns are banned.

That median U turn is way too far from the main intersection.

Also how much is that rotary used?  You don't want to send more traffic to use the rotary if it results in the rotary overloading.  Which the view here shows how adding those additional vehicles could be problematic since there's already evidence that the rotary can jam due to the number of vehicles going towards the bridge.  Or is that due to traffic backing up across the bridge?

[hbelkins]

The time has come that we seriously need to start drug-testing engineers.

[Big John]

^^ During my last stint as an Engineer, I has to take a pre-employment drug test.

[kphoger]

The time has come that we seriously need to start drug-testing engineers.

How is this not at least as good as Michigan lefts?

[kernals12]

One change I would make is to add slip lanes for traffic getting onto the parkway. That ensures that people going left don't hit a red light more than once.

[jeffandnicole]

One change I would make is to add slip lanes for traffic getting onto the parkway. That ensures that people going left don't hit a red light more than once.

Ped/Bike Safety advocates like it this way though.

[kernals12]

One change I would make is to add slip lanes for traffic getting onto the parkway. That ensures that people going left don't hit a red light more than once.

Ped/Bike Safety advocates like it this way though.

People walk across this road??

[kphoger]

People walk across this road??

There are crosswalks that connect to nothing.  However, that was only half of what J&N was talking about.