Friday, October 26, 2012

Unbunching the bus, part 2

Last time I discussed some of the reasons that buses bunch. Here's some possible ways to help alleviate the problem, if put into practice:

Fewer, higher quality bus stops, with curb extensions


Bus stop consolidation or elimination is an important step: it reduces the fixed costs associated with stopping the bus. One problem is political: people are often attached to their particular stop even if another one is close-by. But American standards for bus stop spacing are too small by far. Here in Boston there's plenty of examples of buses that stop every block or two, which is self-defeating. I believe the station spacing should be closer to a quarter-mile, and not less than 800 ft.
Waldo Terrace stop in Brighton Center: barely 30 feet long!

The other problem with bus stop consolidation is the higher strain placed on the remaining ones. This can be accommodated by using money saved to improve those stops to handle the increased load. There are many cases in the city where this could be applied. For example, take Waldo Terrace in Brighton Center. The following two stops are each separated by a mere 350 feet. Waldo Terrace stop itself is only 30 feet long at best, which is far below the MBTA standards of 60 feet for a corner stop. There is no physical way for a 40 foot bus to pull into this stop when cars are parked adjacent to it. So in my experience, the drivers simply stop in the travel lane.

Stopping in the travel lane has the benefit of eliminating clearance times from the delay equation. The problem is access: the curb is too far for level boarding of any kind, and stepping up into the bus does increase dwell time. The answer is to consolidate this stop with the other two, and to build a "curb extension" or "bulb-out" at the chosen site.


Curb extension: before and after (source: TRB Bus Transit Capacity manual)

The curb extension offers a place for passengers to comfortably wait, it brings level boarding to the side of the bus, helps disabled passengers, eliminates clearance times, and it even frees up some curbside parking space. The cost is a small delay to vehicles behind the bus. But the way I see it: that delay is already happening because of the poorly designed existing stations, and it doesn't really cause a problem. Also, the curb extension (with level boarding) can greatly speed up bus boarding time, making it less of an issue. And, the transportation planner should be thinking of delay in terms of person-minutes, not vehicle-minutes. The bus, with its capacity of 50-80 people, should weigh much more heavily in the mind than the typical private automobile, with its average of one or two people per car.


Bus lanes and signal priority


On wider roads, one potential improvement is a dedicated bus lane, which eases a great deal of the variability that makes scheduling buses a headache. They would still require grade crossings at intersections, so a bus detector could be employed to shift the priority of the traffic signal on approach. Bus lanes go well with curb extensions and also eliminate the one downside of curb extensions because there shouldn't be any other vehicles waiting behind the bus. Bus lanes can also provide an outlet for emergency vehicles that are stuck in traffic jams.

Better boarding and fare payment


Going along with those improvements to bus stops should be choices made to optimize boarding and alighting times. That means purchasing vehicles with low floors, wider doorways, and using all-doors boarding -- which most likely implies deploying Proof-of-Payment and possibly pre-payment of fares. A whole other post could be written about this, so I'll leave it aside. Designs made in accordance with the Americans with Disabilities Act (and the Mass. AAB) are also generally good for improving boarding and alighting times and therefore are good for transit service.

Self-stabilizing headways


A while back, Bartholdi and Eisenstein authored a paper named "A self-coördinating bus route to resist bus bunching" (summarized here). They described a fairly simple technique for bus dispatching which can be shown mathematically to reinforce stable, equal, and naturally-arising headways as buses circulate along a route. The gist is that it works by delaying buses at strategically chosen "control points" which are stops along the route designated for special treatment. The most natural choices for control points are the termini of a bus route, where the bus must layover anyway while the driver changes or takes a break. There could be other control points as well. I noticed that VTA designed layovers at intermediate stops along its longer routes, for example (whether they made good decisions on this is another matter).

The Bartholdi and Eisenstein idea is easy to implement: the amount of delay that a bus should undertake at a control point is equal to the backwards-headway times some adjustable factor (which be a fraction between 0 and 1). That factor (called α) must be determined through empirical observations and agency policy. But the backwards-headway is easy to measure, especially these days with GPS transponders installed in every bus. Just check how far away the next bus is from arriving at the current stop, and figure out how many minutes that is, and you've got the backwards-headway.

The neat thing is that you can show, mathematically, that by using this technique, headways stabilize naturally towards equal spreading of the buses. You don't even have to know the headways in advance -- the system will naturally tend towards the ideal spacing. They implemented it for a few experiments which seem to bear out their findings. The main failing, as far as I can see it from playing with the model, is that it may take too much time to unbunch buses if there are not enough control points. The other "problem" is that it is incompatible with scheduled arrivals by nature, although for a frequent bus route the schedule is usually wrong anyhow.

Wednesday, October 24, 2012

Unbunching the bus, part 1

I just wanted to discuss a few potential improvements to bus service here that occur to me as I am riding nearly every day. One of the most notable problems is that of bus bunching: where headways break down and several buses of the same route come along in short order. Probably everyone who has ridden the bus has experienced the frustration of waiting an inordinately long time only to see two, three, or even more buses of your desired route show up together, sometimes all of them busy.

Why do buses bunch?


Unfortunately, there is a natural tendency for buses to bunch like this -- almost a "gravitation" effect which is caused by a combination of factors. The first is the reinforcement of delays due to boarding and alighting. When a bus begins to fall behind, then at each bus stop it starts to face larger crowds than otherwise. These larger crowds require higher "dwell times", causing the bus to fall further behind. What's worse is that the more passengers there are riding the bus, the longer it takes for additional passengers to board. There are some scientific studies of this effect, but it should be fairly intuitive to the observant: just watch next time as riders attempt to board an already-crowded bus and you'll see that it takes much longer to get moving again. Also, the more riders on board the bus, the more likely the Stop Request will be pressed and the less likely the bus will be able to skip any stop (this also makes dynamic "non-stopping" or "expressing" less effective).

How two buses end up leap-frogging each other under heavy demand.

Ultimately the delays may add up so much that the next bus arrives and passes the crowded one. Unless passengers have opted to wait for the next bus (for example, by consulting their smartphones), it is likely that the new bus will be relatively uncrowded and unencumbered, making it much easier for it to catch up. But once the new bus passes the delayed bus, then it starts to encounter bus stops with higher than expected passenger loads, and begins to slow down in the same way. So it begins to "gravitate" back towards the first bus -- which may be moving more smoothly by now. Some of this could be avoided by sending the bus "express" but then that must be communicated to passengers on-board, giving them plenty of time to alight if necessary. On a bus it seems that delay cancels out any gain from going "express."

Clearance time adds random delays

Other sources of delay include increased "clearance time" caused by failure of passing motorists to yield to the bus that is merging back into the travel lane. Although the number of passengers riding isn't directly connected with increased clearance times, there is an indirect relationship: the more often the bus has to pull out of traffic, the more likely it is to spend an inordinate amount of time merging back into the lane. Also, times of heavy automotive traffic -- preventing the bus from merging back -- often correspond to times of heavy bus ridership (e.g. rush hour).

Optimizing traffic signals for cars can hurt bus performance

Finally, I would like to mention traffic signal timing. Traffic signals act as a capacity regulator insofar as they only permit bus movements a limited number of times per hour. Generally, that limited number is high enough to allow the needed capacity. But the timing of lights along a corridor can wreck bus schedules when poorly coordinated. For example, a set of signals that is optimized for automotive flow will probably not serve buses very well, because buses travel in a different pattern from private cars -- instead the bus will find itself stuck at every red light along the way, leading to delays that will probably tip over into the dwell-time "gravitation" trap described above.

What can be done? It seems that the root cause of the problem is the variability of dwell and clearance times. These also happen to be areas where train service is typically superior to bus service; although there is nothing inherent about buses which forces this distinction. The difference is simply that most train designs allow fast boarding and alighting (not the Green Line sadly), and that they do not have to deal with the clearance time issue at all. So, the first place to start, I believe, is to find ways of bringing these advantages to bus service as well.

Continued in part 2.

Saturday, October 13, 2012

Mode shift means more than vehicle choice

I sometimes hear people tell me that they'd love to get rid of their car but they must keep it around for a particular sort of trip. While this is a reasonable complaint on one hand, on the other hand, there is more to switching from cars to public transit than just changing your vehicle from automobile to bus or train. Transit is not intended to be a direct replacement of the private car.  The actual alternative to driving is walking. Transit should be considered as a "walking enhancement."

When walking, your personal geography of the city changes. By that I mean, the outlook you have when consulting your mental map of the places you go and the things you need to do around town. Those people thinking about making the switch from private car to walking and public transit sometimes compare their trips under each case. For example, you might open up your web browser to your favorite maps website and compare the Driving directions to the Transit directions. Oftentimes this comparison shows that the transit alternative takes longer. This causes some folks to balk. But it's not the right comparison to make.

For one thing, driving directions never include time to find parking (which can sometimes take longer than the trip!), and they only occasionally factor in traffic delay. But I'm talking about something deeper: the choices you make traversing the city on foot are different than the ones you make while driving.

Some choices are obvious: you look at options that are closer to home; you frequent local business; you don't expect to carry too many things. You don't make trips out to the giant mall complex by the highway unless you really must. You keep an eye on what's available near stations, bus hubs, or along frequent transit lines. You look for clusters of "microdestinations" where you can quickly walk between the places you need to go. Distance and time projections are malleable: places that are more difficult to access seem further away, while those easier to access seem closer. The quirks of your local transit agency play into this: mistakes made in network design inflate travel time, while well-functioning connections decrease it. (This applies to road networks as well).

Not everyone can handle this change of viewpoint, and there's nothing wrong with that. But for those who are considering the change, it's not sensible to try and compare trips directly. You're not going to follow the same patterns of travel on foot as you would with a car. You're not going to run out to the faraway mall for small purchases, and attempting to duplicate that experience using the bus will likely be highly dissatisfying. You will factor transit accessibility into your decision about what jobs you take and where you live. For example, I have heard one person tell me that they chose one job over another because the other would require the purchase of a second car, which is very expensive. I know that lots of people choose to live near the T because it opens up those options for them.

This observation works both ways: buying a car means that you are going to shift from frequenting local walking-oriented places to those sprawled out on the highway, where you don't have to contend with limited parking. You'll choose to live and work somewhere with easy highway accessibility if that's what's important to you. It's not possible to separate decisions about lifestyle from land use and transportation. They are inextricably tied together, regardless of mode.

I believe that a lot of woes of city planning over the past century can be traced to the understandable desire to have it both ways: catering to vehicles and walking equally. But it's not really possible, and the result can be the worst of both worlds: sprawl with dangerous roads, yet a constant "shortage" of parking. Dis-investment in communities, followed by misguided "urban renewal" and the destruction of cities.

Everything I've said is a corollary of city geometry: shorter distances are more amenable to people on foot, but provide fewer places to conduct and store bulky vehicles; while large distances provide ample space for those vehicles, but are an obstacle for walking. This all may seem obvious, but for some reason, I find that it often gets overlooked.

Monday, October 8, 2012

Free highways lead to gridlock in China

The idea of pricing road usage is still controversial in this country. Although it would likely save everyone some time and headaches from congestion (and time is money for most folks), people resist paying for access they feel entitled to be given for free.

This year, China recently decided to celebrate a week of holidays by making all roads free for travel. The result was epic gridlock.
Long tailbacks were reported across the country, with 24 major motorways in 16 provinces effectively transformed into enormous parking lots as 86 million people took to the roads, a 13 per cent increase on last year.
Not everyone was surprised:
Li Daokui, one of China's most prominent economists and a policy adviser to the Central Bank, said the snarl-up was entirely predictable. 
"[Making the] Highways toll free on holidays? We are making a world record of stupidity by launching this policy […] Going free of charge is like shouting out to the public: '1,2,3, let's go jam the road!'" he posted on his Weibo.
I'm sure though that, as an economist, he may have some small appreciation of the mess as an unintentional experiment. I think that this decision by the Chinese government, regarding their toll roads, does a good job of making the case for road pricing: by demonstrating that the opposite causes chaos and unbelievably high levels of congestion.

Monday, October 1, 2012

Barry's Corner Parcel A


View Larger Map

Harvard, the BRA, and the hired developers came by to discuss plans for Parcel A in Barry's Corner. That's the one on the northwest corner of Western Ave and N. Harvard St. I wish I had some pictures to show, but they don't seem to have posted them online. In any case, the plan is to bring activity and diversity to the corner. The building will be mixed-use with 40,000 s.f. retail first-floor and residential above. It will range in height from two, to six, to nine stories. It seems they are leaning towards fully underground parking. There will be a new street created, Grove Street, along the northeast edge of the building, between 175 N. Harvard St and the new building. One slide placed the corner in the context of public transportation, with the 66, 70, 86 and Harvard Shuttle shown. Upgrades to transit access, they say, are forthcoming. They do plan to alter the Harvard Shuttle's route to take advantage of one of the new roads, and eliminate the extremely sharp curve it currently has to navigate.

So far I'm liking what I'm seeing, for the most part. Still need to see specifics in the upcoming institutional master plan PNF, but the presentation was good. All of the people in the room seemed to agree that Harvard did a good job this time with the presentation. They were also happy to see some changes made in response to comments earlier in the summer. Unfortunately, one of those changes was to reduce the maximum height from 11 stories to 9 stories. I'm not quite sure what's driving the community opposition to height, except that it might be rooted in some desire to see Harvard Stadium be the tallest structure in the area. Admittedly, I personally find this to be a weak reason. Anyway, one person asked if going to 8 stories was a make-or-break request. It strikes me that this was testing the developers. If they were willing to concede 8, then would someone else come forward and request 7? This could get out of hand.

There was a long discussion about the economics of building a project like this. The developers are interested in 275-325 units, and have proposed 300 for the time being. So far, they are holding firm on that, which is a good thing. The parcel is 2.4 acres so that works out to about 125 dwelling units per gross acre, which is a decent density for a city block. Interestingly, one of the finance guys spoke about some of the economics of the plan, saying basically that it cost 30-50% more money to build in the city than outside of it. He said that this construction cost is the main driver behind the cost/benefit analysis. He also claimed that pretty much all development in the city has razor thin margins in the first year, but the goal was to achieve decent profitability in the long run, over the course of decades.

Harvard envisions that the blocks surrounding Barry's Corner will also grow up in similar fashion, outside of the preserved open space areas. They showed some of this in several conceptual art images. Although you do have to take those visions with a large grain of salt, I do hope they're right.