Wednesday, February 20, 2013

Direct Service & Express

Express to the U

Earlier, I wrote about the attractive potential of starting a ride-share of 'caro publico' between the Salt Lake Central Station and the U. While it looks like I was right in predicting the demand for quick passage between the station and the U, I was wrong about the mechanism. Rather than an on-demand service to the hospital, there is a enough demand for a shuttle. 

The 'directness' seems to be the important part. It may be useful to start thinking about high-capacity transit in the same way we think about air-lines--as rapid transit between hubs, rather than as service along a route. Certainly, it seems like a waste of time not to pick up passengers along the route, but it's a matter of aggregate delay. Stopping a shuttle with 20 people on it 6 times to pick up six passengers (even if the stop is only 10 seconds) is still 20+ minutes of delay. 

But the power of a shuttle is that it can do just a little more than a plane (which is hub to hub). You can also have some stops 'after' the end of the 'direct' route, so you can distribute passengers around an area. It's a stopping pattern I'm seeing a lot in light rail---a long 'direct' section, and then a large number of stops downtown. 

Direct:  x--------------------------x
Direct Plus:  x-----------------------------x--x--x--x
Local: x--x--x--x--x--x--x--x--x--x--x--x--x--x--x
Rapid: x----x----x----x----x----x----x----x----x 
Too many stop: x-x-x-x-x-x-x-x-x--x-xx-x-x-x-x
(Presume each dash is 660'---a SLC block.)

I increasingly believe that a great deal of the value of fixed-guideway transit is coming from stop spacing. Expensive stations means fewer stations, which means fewer stops,which means better speeds, which means people are willing to walk further (or wait longer).

Tuesday, February 12, 2013

Hex City II

SimCity hex-grid traffic plan.

The blue and green streets are one-way streets in the direction indicated. The red and orange are two-way streets, with a wrinkle--on the orange roads, the you drive on the left, rather than on the right (with a visual barrier between, to prevent confusing drivers).

The result of this complicated pattern is that every single turn you make on the network is a 'safe' turning movement that does not require crossing traffic, so there is no-need for stop-signs or traffic lights, only terminating turn-pockets onto and off of roads. Consequently, you can go pretty much as fast as you like.

The turning rule is:  You never cross traffic. So you can go left from green onto yellow, and from blue onto yellow. The rule ought to have been 'rights onto red, lefts onto yellow'.

Travel in the NW-->SE direction(blue, green paths) is fast direct. But travel in the NE-->SW is a pain (gray path), because every other road you have to take a 'red' street, and wind up going about 50% more distance. I think the lack of streetlights would be worth it, but not sure. The plan would be most suitable for a city where most of the travel is in one direction, such as a mountain valley. 

I can't see retrofitting an existing city, but it would make a novel subdivision plat, or the  traffic plan for a tabula rasa city like Brasilia or Masdar.

 For local streets, connecting these main-streets, you can do pretty much what you want, with some limited exceptions: The turn pockets have to be oriented toward the direction of flow,  so as not to generate temptation to 'cross' in front of traffic, on red or yellow streets:

  • practical problems: will ignore drunk drivers, but how about people who aren't spatially inclined? I could imagine getting lost *extremely* easily. 
    I don't propose to make this real in the least--it's a bad idea in a multitude of ways. It's a traffic network for SimCity, or for Traffic Simulation software. If you are working with a 'omnipotent' traffic allocator, which knows the whole network (and never gets lost) it just chooses the most direct route for each residential cell and the nearest employment cell.

    A system like this would drive most people nuts. They would have to constantly be aware of where they are because if they miss a turn, its much more costly, timewise, to correct it than on a square grid.
    Most places don't actually HAVE grids, or ones of analogous scale. Grids are more the exception than the rule, actually. Phoenix has one due to it's near-total lack of geography (rivers, mountains, valleys, etc).

Hex City III

  •  With such a street network, you can do some funky things with the signals. For example: Use three-phase signals, so each of the 3 directions are 'green' in turn, for right or left turns. Or put a roundabout at each intersection. You could also make some of the legs one-way streets...
    SimCity Traffic network 2 - trinary. More geographically uniform, with three directions instead of 2.
    The signals alternate by street color. For example: At a given intersection, on red's 'turn': Cars on a red street may turn left or right, cars on a green street may turn right, and cars on a blue street may turn, but must yield. Then it's blue's turn, then green's, then red's again. 
    At any intersection, 4 of the possible 6 turning movements are being made at any given time. Traveling in as straight a line as possible, a driver will have to make a left 50% of the time. At a given intersection, 33% of the time they will immediately be able to make a left. So chances of stopping (for a straight path) at any given intersection are about 1 in 6. Half of those stops will require waiting for one color to go, and half for two colors to go. 
    Single and double lanes only. The grid-scale is analogous to secondary arterials, at mile to half-mile spacing. 
    High-traffic places would have traffic arrows to tell you when you can go. Medium traffic places would have a two or 3-way stop, and low-traffic locations would have zero or one stop-sign. 

Monday, February 11, 2013

Hex City I

New SimCity coming out. Hopefully it will be at least as good as the first one. Early indicators are not positive...

But it inspired some interesting comments...

"Could you imagine driving in a city layed out like that? every 100 feet a stop sign or traffic light. On the other hand, everyone has a park in their backyard"

Yes and no. With such a street network, you can do some funky things with the signals. For example: Use three-phase signals, so each of the 3 directions are 'green' in turn, for right or left turns. Or put a roundabout at each intersection. You could also make some of the legs one-way streets...