Tuesday, June 12, 2012

Road Width vs Road use

When I was researching information about the new green housing development in Brighton I found some interesting numbers regarding the traffic on Commonwealth Avenue. As part of their permitting with the Boston Redevelopment Agency, a big PDF details the traffic patterns in the neighborhood, for both pedestrians and vehicles. Looking at the traffic numbers, I got the feeling that Commonwealth Avenue in Brighton is not equitably sized. In other words: the width of the road is not proportional to the number of users for each segment.

Here's how the roadway in question breaks down (I measured it online):

The unlabeled gray sections are medians. Here it is simplified by use:

And here's a pie chart of how many feet (the total width is 200 feet) are used for each use:

About two thirds of the street is taken up by traffic lanes, parking or medians which separate traffic lanes and parking (and provide no refuge to pedestrians and no landscaping). But of people traveling along Comm Av, fewer than two thirds are traveling by car. Many fewer. According to the study, there are, at the peak PM rush hour between 5 and 6 p.m., 1343 vehicles traveling along Commonwealth or turning on to or off of Harvard. Of these, 785 go through, and the rest turn. Counting each turner as half a trip, there are 1064 road users along this stretch of Commonwealth Avenue. Assuming some carpooling, this probably equates to about 1400 people per hour.

There was no count for cyclists. This is a frequented stretch of road by bikes, but it is certainly not bike-friendly. I'd guess that there are 50 bikes per hour in total at rush hour. (There are no marked lanes for cyclists and they have to choose between the trafficked main travel lanes or the carriage/parking lanes which have parked car hazards and more stop signs. Most choose the former.)

For pedestrians the counting is easier: there are about 300 walkers per hour.

As for transit users: the T maintains a six-minute headway along this stretch of street during rush hours. In recent years, they've moved from two-car trains to three-car trains, and about half the rush-hour consists along this line have three cars. That's 25 vehicles per hour in each direction. In the peak direction, the T operates at or near crush capacity in this section, with 150 to 200 passengers per car (specs here). The non-peak direction probably operates at about one third that capacity, with 50 passengers per car (around all-seated capacity). This estimate gives us, conservatively, 5000 people per hour.

So, compare the chart above to this one:
The vehicle right-of-way, which uses the lion's share of the street's real estate, sees fewer than a quarter of the street's users. Transit, with less than a sixth of the street width, carries more than triple that number. Per linear foot, the roadway and the sidewalk come out about the same. By passengers per foot of right-of-way per hour (ppfph) the numbers break down as:

  • Vehicle ROW: 11.1
  • Sidewalk: 12.5
  • Transit: 172.0

Note where the decimal is for the transit. It's 15 times more efficient for each unit of real estate.

This can be documented for many other streets. Take the main cross street here, Harvard Street. It sees about 900 cars (1200 passengers) at peak hour, 350 pedestrians and (I'm guessing here) 60 cyclists (it was not counted in the bike count database since 1976). The street right-of-way is 78 feet wide, with 22 feet of sidewalk, 10 of bike lane and 46 for vehicles (travel and parking). Of course, the MBTA's route 66 bus runs every 9 minutes at crush capacity (60 passengers per bus) in both directions, carrying about 800 passengers.

On Harvard, the sidewalks carry 15.9 pedestrians per foot per hour, the bike lanes 6 and the vehicle lanes 43.5. (Not terribly surprisingly, Harvard Street is usually gridlocked between 5 and 6 p.m.) This neglects to account for the efficiency of the buses. Buses demand some real estate, namely a 10-foot-by-50-foot bus stop every 1000 feet or so. That breaks down to one half of one linear foot, but for good measure we'll assume the buses use that much street real estate during travel, and assign two of the vehicle feet to the buses. That changes the numbers:

  • Vehicle ROW: 27.3
  • Bike lanes: 6.0
  • Sidewalk: 15.9
  • Transit: 400
Does that mean that the 66 bus is more efficient than the Green Line? Certainly not. With 800 passengers per hour, the 66 is stretched to capacity: it gets bogged down in traffic, it frequently runs late or in bunches, and it crawls along its route. To add many more passengers, it would need its own lane. Even still, if, as a bus rapid transit, it took over the parking lanes (note: this won't happen any time soon) and doubled its ridership (likely, considering how many people take it even despite its slothly pace, it would still be more efficient than the adjacent roadway.

(And one more note: we'll look at the Red Line on the Longfellow Bridge soon, but the back of the envelope calculation is 15 trains each way carrying an average of 750 passengers each per hour in 30 feet of right of way, giving a ppfph of 750. A New York Subway line running every three minutes with 1000 passengers per train would yield a ppfph of 1333. A highway at peak capacity might be able to attain 2700 passengers per lane per mile—or a bit more with a lot of buses—for a ppfph of 225, but adding any more vehicles quickly decreases the speed and capacity.)


  1. Interesting stuff. I think some of the estimates are a bit off but the general idea is right.

    According to the Blue Book 2010: crush capacity on 40' buses the MBTA uses is 72-77 ppl. The 60' buses are 96-104 ppl.

    By contrast the Green Line Type 7s max out at 269 ppl, the Type 8s at 199, and the Red Line cars range from 260-272 each.

    "Rush hour" on the "B" line is a bit strange. It's common to see crush loaded vehicles even late at night. Blue Book claims that "B" line sees a daily boarding count of 28,486. That doesn't include people who board in the Central Subway and alight on the surface, though.

    P.S. You can measure distance in Google Earth much more conveniently than that website.

  2. I used these estimates for green line trains for a couple of reasons. First, I'm not sure I believe the crush capacity numbers. People in Boston seem not-that-great at crushing in to cars (often full near the doors and breathable in the middle) and since the Type 7s and Type 8s are the same lengths and widths, I don't understand the 35% discrepancy there. The number of boardings in the Blue Book is based on counts from 2006 and 1995; I would imagine the B line has changed somewhat in that time. And, yes, rush hour on the B Line goes all day, but there is a commute peak in the early evening. So, yeah, that's why I guessed.