Sunday 16 February 2014

Why collisions don't occur between cyclists on Simultaneous Green junctions - bikes are ridden through curves not sharp angles

This location, a large Simultaneous Green junction in Groningen, was chosen for this example because of its symmetry, making it easy to illustrate with arrows showing the routes that people take as they cross the junction. To simplify the diagram I have not shown the straight over routes by bike or the legal right turn against a red light routes. (Google map)
In Assen and Groningen we have many Simultaneous Green traffic light junctions. These offer the greatest safety and convenience for cyclists. Cyclists can at any time make a right turn without stopping even if there is a red light, while straight on and left turns are made with cyclists going in all directions at once, separated in both time and space from motor vehicle traffic (all directions for motor vehicles have red lights while cyclists have green), never having to make an inconvenient two-stage turn in order to cross traffic.

Simultaneous Green design for cyclists works at both small and large traffic light junctions. This junction has many lanes of motor traffic, all of which is stopped while cyclists cross in one swift, safe and convenient movement. Junctions like this can also cater for vast numbers of cyclists. Note how the cycle lanes on several sides of the junction have more than one lane to cope with busy traffic. The cycle-path in the bottom right corner is five metres wide and has two northbound lanes combined with a southbound lane.

I drew this diagram in order to illustrate a common misconception amongst people who have never seen a simultaneous green junction in reality. It is often imagined that there will be many collisions between cyclists at the centre of the junction. If hundreds of cyclists really did try to ride perpendicular to each other directly through the centre of this junction at the same time then there would indeed be collisions, but that is not the reality.

Cyclists travel through curves
The distance that A has travelled to
reach the potential conflict point is
much shorter than the distance B has
to travel to do the same. The result
is no conflict in reality.
Cyclists do not make sharp 45 degree or 90 degree turns. Rather, they travel through graceful curves because this is the only way to control a bicycle. All design for cyclists should be made with this knowledge in mind. Sharp angles have no place in cycling infrastructure because they cannot be followed by a cyclist without slowing down.

In the came of simultaneous green junctions, the arcs that cyclists travel through result in the potential conflict points not being reached all at once by all cyclists but in fact being spread through time and space from the point of view of any cyclist using the junction.

Consider cyclists A and B in the diagram. B has to ride considerably further than A to reach the same point. This means that for cyclists riding at similar speeds there simply is no conflict. He will in any case also expect to give way to A because the convention at these junctions is that everyone gives way to the right.

There is no kerb in the corner of a Simultaneous Green
junction. Cyclists must be free to follow many parallel paths
in order to make the junction efficient. Cyclists consume less
of the traffic light green time than with a two stage turn.
What's more, this is a simplification. My arrows do not show the exact lines of cyclist. In fact, the arcs used by individual cyclists at junctions like this vary enormously. While each cyclist turning left will start and finish in roughly the same place, these arrows should really be much wider in the centre.

While the cycle traffic lights are green, the entire area of the road junction is open for cyclists to use optimally. Faster cyclists often overtake slower cyclists while crossing diagonally. Negotiation takes place in that one party sometimes speeds up a little while another slows down. It is also straightforward to take a wider arc and therefore to go behind someone coming from your right.

The larger the junction, the more parallel routes are possible and the more efficient Simultaneous Green becomes both for cyclists and for other traffic as well because their total red time will be shorter with a simultaneous green layout than if cyclists cannot ride parallel and are required instead to follow each other in straight lines and cross twice.

Success !
It's easy to pick up misconceptions from blogs,
photos and videos online. Simultaneous Green
junctions are demonstrated on our cycling
infrastructure study tours
Simultaneous Green junctions are extremely successful. They have very good safety records and scale to all different sizes of junctions.

Where it is not possible to remove traffic lights altogether or to make efficient routes for cyclists which don't include traffic lights for drivers, this is the best solution to make cycling both efficient and safe.

Read more about Simultaneous Green junctions (includes videos showing them in use)


Unknown said...

Would you mind forwarding this explanation to the DfT/TRL, who apparently have no intention to trial a simultaneous green (what I would call a 'scramble' crossing). Evidently, trialling the low-level cycle signals and Dutch-style roundabout gave them an attack of the vapours and apparently they need to recover before even attempting something like this.

seiklmeikl said...

Where are the pedestrians in this approach? Could you explain a bit about them as well?

David Hembrow said...

Andrew: please get in touch with the DfT and TRL yourself and suggest it. The more people who do this, the more likely it is that they'll take notice. Please reference my blog as there's almost certainly more English language information here about Simultaneous Green crossings than you'll find anywhere else.

If they really want to understand how these junctions and other Dutch infrastructure designs work then it would doubtless be helpful for them to send people on a study tour.

M Stoss: Pedestrians have their own paths, separate from the cycle-path, and you can see the pedestrian crossings on the map. On each arm of the junction there are short sections of zebra crossing which go across the cycle-paths and then there is a button operated crossing for walking across the road.

Pedestrians do not have a green light at the same time as cyclists as that would cause conflict. Actually, their greens are synchronized with the red lights for drivers because with the relatively low speed of pedestrians vs. cyclists it is advantageous for pedestrians to be able to walk half way across the road and stop (islands have their own buttons to press to cross further) on a junction so wide as this.

Clark Nikolai said...

Vancouver has it's first all-directions light phase for cycling that went in the summer of 2013.
It's not yet visible on Google Maps or Streetview but you can see the plan here:
Look at the intersection of Main Street and Union Street.

When I saw the plans I initially thought that it wasn't going to work but now I use it every day and it works well and I haven't seen any problems. It's pretty obvious for users of any mode what to do and when.

It's a weird intersection

seiklmeikl said...

@ David: Would this work for a crossing with high traffic volumes of both cars, bicycles and pedestrians in the heart of a big city as well? Looks a bit like this is a good solution for places with mostly cars and bicycles?

David Hembrow said...

Clark: good to see that something new is being tried. The diagram doesn't look like simultaneous green, though. It's the "conflict zone" ? Do drivers have a green to make right turns at the same time as cyclists have a green ? And is there somewhere to stop in the model of the junction ? Why would you do that ?

Further to the left, what's the crossover thing on Union Street before Expo Blvd ? I'd also be suspicious of the way the road narrows at the junction of Carrel Street with Expo Blvd Looks like it pushes bikes out in front of cars.

Further south (down) on Expo Blvd, there are Bike Boxes and a cycle lane which is between two lanes of moving cars. Neither of these should form a part of modern infrastructure design.

Did you read my article about Ontario's cycle design guide ?

M Stoss: Why would you imagine this is bad for pedestrians ? It works just fine.

Manny said...

David: in British Columbia, don't need a green light to turn right. For right turns (or left turns onto a one-way street) a red light is treated like a stop sign -- you must come to a full stop and then may turn when it is safe. In practise, of course, few drivers come to a full stop. There are some intersections where there is an extra "NO RIGHT TURN ON RED" sign, but it's not common. I haven't seen this particular intersection, so I don't know if there is anything like that there.

David Hembrow said...

Oh. In that case the Vancouver design is simply dangerous. Simultaneous Green works in NL because all motor vehicles are stationary while all cyclists have greens in all directions. Separated in time and space. No conflict and no danger at all

Clark Nikolai said...

Oops. I didn't finish my sentence there before it got posted. The last sentence should read:
It's a weird intersection because of a viaduct so there are two different streets you could aim for when going west. The viaduct is a leftover from a halted downtown freeway project of the early 1970s and is slated to be removed sometime in the 5 - 10 years. This intersection will change then to something more conventional.

Drivers cannot turn right when the cycling light is green. Also not turn right when they have a red (which is normally allowed at intersections here.)
The cycling light phase is for all directions, cycling only. (I think the north crosswalk is also lit for some of that time though.) All motor vehicle lanes have red lights during this.

The island in the middle is not for cyclists to wait at, it's to prevent northbound drivers from making a left turn onto Union st. There isn't any waiting and the cycle light phase is generously long.

The crossover thing at Expo and Union is a way to merge a two-way cycle path with two one-way cycle lanes. The motor traffic there is very low because it's closed to motor traffic further east at Main. This is another thing that will change later when the viaducts are gone.

I agree with the weird narrowing of Carrall where it meets Expo. Conflict at the north west corner with right turning cars is common. (Carrall St. was made into a greenway several years ago and is of an older design.)
I would think that the cycle lanes really should curve away from the motor vehicle lanes at intersections instead. Would you say so too?

Further south on Expo and Quebec, that's an older cycle lane between the turning lane and the curb. I used to take this and had to be brave but I no longer take it as there is now a better path west of it. I don't know why this one wasn't upgraded when they did the rest.

I did read your post about Ontario's cycle design guide. I agree with your points. Ontario is going in the wrong direction. Maybe it's the automobile industry there or something interfering, I don't know. They have it tough there. Toronto is an excellent place to cycle, flat, small downtown area, etc but their mayor and some councillors are outright hostile to any bit of pennies spent on cycling.
It seems that every type of government is now expected to produce something like this report whether or not they are truly interested in encouraging or even if they want to discourage cycling. I suspect the authors of this report, while possibly well intentioned, just can't imagine that things could be any different than they are now.
Here's a cycling design guide from the British Columbia Capitol District. It seems to be much better, (though it still includes things that in my opinion should just simply never be made anymore.)

David Hembrow said...

Clark: Thanks for the clarification. If no cars move when the bikes have greens then this sounds much more similar to the Dutch junction, also much safer than if right turning cars could conflict with cyclists. That's very good to hear.

All the cycling design guides that I've read are lacking in one sense or another. Even the CROW guides give too many choices and use such passive language that you can't tell from reading them what should be built, and they aim at the low side on their minimums (Dutch infra usually exceeds their recommendations)

Clark Nikolai said...

Here's a photo of the intersection.