Thursday, 4 February 2016

Simultaneous Green

In the last section regarding the basic options for signalized intersections, I will talk about simultaneous green.

In the last section I talked about protected intersections, so let's see what those look like:

Not too difficult to understand. Now with simultaneous green, at least modern ones, the corner refuge island is actually open, with the cut throughs for cyclists closed. Lets see what it looks like: 

Also note which motor vehicle lanes can go in which directions. Before I had the left turn and thru lane mixed, here I have the right turn/thru lanes mixed. This is more usual, but this design will do better if we had a turn lane for both the left and right turns. Protected intersections also usually have the dedicated left turn lane and a mixed right turn/thru lane if there aren't separate lanes for each direction, the latter is more preferable and the Dutch usually consider the left turn head on conflicts as more dangerous than the right turn/bicycle/pedestrian conflicts. 

Pretty simple signal staging. Simultaneous green, thru and right turns WB-EB, then left turns WB-EB, plus right turns NB and SB, and then the other direction car stages again but rotated 90 degrees to the right or left. Lets assume 18 seconds for the SG stage, 10 seconds for the left turns and 18 seconds for the thru. Cycle time, 74 seconds, right in the ballpark. Cycle times between 60 and 90 seconds is ideal. Considering that our current cycle times including an 8 second advance green for the left turns and 30 seconds for regular thru movement is 76 seconds, we actually do better. And in a way that no motor traffic has conflicting greens with either pedestrians/cyclists or with other motor traffic, it is more likely that fewer people are going to be very dead or injured as a result. 

Pedestrians do not encounter issues with simultaneous green intersections. They can divide their crossing into easy to follow steps, most people can cross within a single walk stage, their crossing is direct and you can deal with cyclists in a separate step from motor traffic, plus you get to cross without turning traffic being allowed to conflict with you. You don't cross diagonally, you cross like you would at any other intersection. 

By having paths that are wide enough, 2.5 metres normally for one direction paths, 3.5-4 metres normally for bidirectional paths and 3.5-4.5 metres of width where the cyclists actually leave their protected space on a green light, there are few conflicts on the cycle paths. 

The cyclists don't converge at a single point. They spread out the conflicts over a larger area, reducing the chance of a collision, and because different cyclists will be at different points at different times, collisions are rare, injurious ones even rarer. 

Cars have to be prohibited from turning right on red for this to work (they do at all intersections but here it is particularly important). You can't have cars making right turns with all the cyclists going every which way. And cars cannot have the same green light as cyclists. They CANNOT. It will fail and many will be injured and many collisions will happen, I can almost guarantee that. 

This is a successful type of intersection design that works under all weather conditions at a wide variety of intersection types of differing volumes and speeds, and even works at asymetrical intersections. It even helps with subjective safety because even with separate signal stages parallel moving traffic can still intimidate to some degree. Why not adopt it as a primary way of handling signalized intersections? 

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