[nostrademons]

> No more hierarchical/dendritic street layouts.

It's worth playing some Cities:Skylines (which has a fairly accurate traffic simulator, particularly with the Traffic++ mod) to understand how the road hierarchy came into being. Or, for that matter, trying to drive through a grid-based city like Manhattan or SF.

You get very large traffic jams. The problem is intersections, and particularly intersections where traffic backs up to the previous intersection. When this happens, a traffic jam tends to spread across the whole city; incoming traffic can't clear the bottleneck fast enough, so the bottleneck just grows like a cancer until it envelopes a whole neighborhood.

Oftentimes, the solution to a traffic problem is simply to bulldoze a few intersections. By doing this, you give cars a buffer. It increases the median trip length but it also increases vehicle speed and road throughput by more. It turns out that the major contributor to traffic jams is the acceleration of having to start/stop at traffic lights and when turning.

Self-driving cars (or just ubiquitous turn-by-turn navigation) could change this equation by intelligently routing cars around bottlenecks and avoiding the neighborhood entirely, but as long as drivers have imperfect information about traffic conditions and tend to take the shortest route to their destination, this will remain a problem.

(I've had great success with using pedestrian paths to provide cut-throughs between dead-ends and nearby intersections, though. And with providing pedestrian paths under or over those intersections so that people don't have to wait for stoplights to cross the street and don't stop traffic with their jaywalking. The game unfortunately has pretty terrible pathfinding for pedestrians and won't let you build compact staircases, so this limits their usefulness to real problem intersections, but in real life I think many suburban cities could drastically improve their walkability/bikeability just by building raised pedestrian footbridges over their major arterials.)

[thescriptkiddie]

I really enjoy C:S, but even with Traffic++ the traffic simulation is pretty wonky and shouldn't be taken as reflective of reality.

But here's some advice if you're having trouble with traffic jams in your grid systems: use more one-way streets. If you've converted your city over to a 100% one-way grid and still have backups, you probably need to work on your mass transit and freight rail systems. I've made functional cities where every single road was open only to pedestrians, cyclists, and service vehicles. No cars.

[malyk]

According to Jeff Speck (https://www.amazon.com/Walkable-City-Downtown-Save-America/d...) 1 way streets have a negative economic impact for the business that line those streets. Which I think makes intuitive sense. Anyway, that doesn't matter in C:S, but it probably matters in the real world.

[syrrim]

Is this assuming all streets are one-way? If they are, than I wouldn't expect an economic impact on businesses. Since the amount of products people buy - and the money they are willing to spend - should stay the same, than the total money influx would remain constant. Therefore, the only change would be how this money is distributed. Since all stores would lie on one-way streets, none would be more impacted than the other.

[malyk]

It's definitely about the specific businesses that are on the one way street. And I might recall (been awhile since I read the book), that he specifically used the example of changing from two way to one way and that the result is businesses on the affected street lost revenue.

It makes intuitive sense. When a street is two way cars pass by both directions each day increasing the likelihood of stopping at a store on that street. When streets are one way, then stores only get people driving by once a day, and so they lose revenue.

[prawn]

Something that also doesn't help with a lot of intersections is dense street parking (blocks line of sight for approaching traffic). It's problematic in cities and getting worse in suburbs. The street I grew up on, was generally empty of parked cars making it a great place to ride or skate in the 1980s. Now, that same street has loads of parked cars - people have second cars, use the street instead of their garage, or their adult children have cars.

Roundabouts would also drastically help many US cities currently relying on all-way stops.

[pc86]

Isn't a large contributing factor to this people being in the intersection ("blocking the box" in NYC) when the light turns red, causing literal gridlock? People, myself included, have a visceral aversion to sitting at a light as it cycles through and not moving, so the instinct is to move forward even if you're going to be sticking into the box a little bit. Then the person behind you does it because they've been waiting just as long as you, and suddenly the perpendicular traffic can't move at all because you're blocking their green light.

I always thought that was why I saw posted fines for blocking the box in NYC but never saw such signs elsewhere (although I've never driven in California).

[coryfklein]

> Self-driving cars ... could change this equation by intelligently routing cars around bottlenecks and avoiding the neighborhood entirely

Self-driving cars can do even better than that - they can eliminate the "stopping" nature of the intersection entirely. If the self-driving cars are able to determine the position and speed of other cars approaching the intersection, you can just have the traffic streams pass right through each other. [1]

[1] Autonomous Intersection Management: Traffic Control for the Future https://www.youtube.com/watch?v=4pbAI40dK0A

[zephjc]

I gotta agree with thescriptkiddie, C:S is an imperfect model of how traffic behaves.

> Self-driving cars (or just ubiquitous turn-by-turn navigation) could change this equation by intelligently routing cars around bottlenecks and avoiding the neighborhood entirely, but as long as drivers have imperfect information about traffic conditions and tend to take the shortest route to their destination, this will remain a problem.

Self-driving cars or drivers with good mapping are still limited where they can go when they have a street hierarchy to deal with, forcing all cars onto the same few arterials.

> Oftentimes, the solution to a traffic problem is simply to bulldoze a few intersections. By doing this, you give cars a buffer. It increases the median trip length but it also increases vehicle speed and road throughput by more.

This is all well and fine in a game, but increasing street speeds kills the street life (figuratively, and sometimes literally). Slower but more constant speeds are better for everyone involved. For walkers, bikers, and even drivers. Ask yourself this: would drivers flip their shit more often when going slow but steady down 15-20 mph hour streets with stop signs (or roundabouts), or when they're stuck at long traffic lights regardless of how many lanes they have?

[nostrademons]

The benefit of faster speeds isn't driver convenience, it's that you get cars off the road quicker for a given travel distance. Each city has a carrying capacity for the number of cars that may be on its roads at once, which is determined by the length of the road network and number of lanes. They also have a certain number of trips generated, which is determined by population. The number of cars on the road = trips generated * average travel time per trip. When that exceeds the carrying capacity of the city, average travel time increases, which causes a cascading effect that eventually results in gridlock.

The same effect plays out locally, on each individual stretch of road. When integral(# of incoming cars - # of outgoing cars, time) > carrying capacity of road, the road backs up, which increases the time required to traverse it, which further exacerbates the backup. This is why multi-lane arterials can reduce congestion; they can move a lot of cars off a given stretch in a short period of time, and provide a linear buffer where momentary oversupplies can collect without backing up the previous intersection.

You can also see this effect by looking at traffic maps of say, SF (grid layout) vs. Sunnyvale (arterial/collector):

https://www.google.com/maps/@37.7811106,-122.4106957,16z/dat...

https://www.google.com/maps/@37.3724565,-122.0375532,15z/dat...

Both of them have shitty traffic, but in SF the traffic spills away from Market street onto many of the side streets, such that no matter where you go it'll be gridlock. In Sunnyvale, much of the traffic is confined to major arterials like El Camino or Matilda, which are slow but still move, and side-streets that parallel them are often relatively clear.

[zephjc]

> The benefit of faster speeds isn't driver convenience, it's that you get cars off the road quicker for a given travel distance.

Yes but that isn't actually what happens because the use of a street or road isn't by one car from A to B but by the continuous use over time across a section of the street.

Say you had a single 1 mile arterial in a city, and it's the only way of getting from one half of the city to the other half. There are few points when a cars are "off" of it (except maybe late at night) - the rest of the time it is a near constant high speed flow.

You're not wrong to say that it gets any given car off the road quicker, but that is if you're focusing on the one driver's trip, as opposed to focusing on the use of and experience of being at that section of road. If it was an old town which had its main street become a high speed arterial, you now have an experience for any pedestrians who might want to use the (probably few remaining) stores along that road be not unlike walking along side a freeway - unfun and dangerous.

By focusing on any one driver's trip experience, and not the street experience, you're essentially damning the street experience for the potential sake of some extra time saved (if its across a city, perhaps on the order of 10 or so minutes).

Of course when you have nothing on the street worth being around (like most of El Camino Real), you want to get passed it ASAP. (SF problems are a whole other hairball of outside commuters plus residents who insist on using cars.)

[delinka]

Did you intend to paste the same quote twice?

[zephjc]

I did not, thank you; fixed :)

[greggman]

Tokyo has many intersections with over/underpasses. The 2-4 lanes in the middle go over or under the intersection. The outer lanes connect to the intersection.

That said Tokyo has plenty of traffic.