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Friday, January 27, 2017

BRT vs. Traffic Lane

Back of the envelope calculation here:

For a BRT:

Assuming an articulated bus is purchased, passenger capacity per bus can be estimated at 90 passengers for an articulated New Flyer vehicle[2], or 108 for an Xcelsior vehicle. With 5 minute peak headways, this equates to 12 buses per hour per direction, or 24 buses/hour total. With a potential of 90 passengers per bus, the Provo-Orem BRT would have a capacity of (90*60/5*2) is 1080 passengers per direction per hour. For the Xcelsior, the Provo-Orem BRT would have a capacity of (108*60/5*2) is 1296 passengers per direction per hour. The IRIS Civic Bus, used for the Las Vegas MAX, has a capacity of 120 persons; peak hour capacity in which case would be 1440 passengers per direction per hour.  

So how does that compare to a highway? 

Max capacity per lane for automobiles is 1900 per hour, says "Mike on Traffic'. Table 31 of the "Default Values for Highway Capacity and Level of Service Analyses" suggests this is a reasonable number.

So the BRT (at max capacity) is less than that for freeway lane. Damning, eh? Not quite. That's BRT capacity at 5 minute headways, or 12 buses per hour. BRT is suggested to cap out at 17,000 per hour.

But I'm skeptical. How good a source is Marin? Assuming 17,000 is both directions, with a capacity of 120 per bus, that's 71 buses per hour. That's a bus every 50 seconds. That strikes me as unrealistic.

Ontario suggests a somewhat lower number, more like 5k (bus in bus lane), in one hour, in one direction. So that's more like 10,000 in both directions, rather than 17,000. 5,000 passengers per hour at 120 passengers per hour is about 42 buses per hour, which is a bus about every minute and 45 seconds. That seems more feasible.

But we are talking about Provo here, so let's ignore the 'theoretical max' and talk about the specifics. Assume the midpoint (the Xcelsior at 108 passengers) rather than the CivicBus at 120.

For the Xcelsior, the Provo-Orem BRT would have a capacity of (108*60/5) is 1296 passengers per direction per hour. If we increase that to a bus every 4 minutes, we get 15 buses per hour, with a max capacity of 1620 persons per direction per hour (peak capacity). Following that logic, we can generate the following chart:

Headway------- Buses per hour-----------Capacity
        5 min                     12                              1296
        4 min                     15                              1620
       3:30 min                 17                              1836
       3:20 min                 18                              1944
        3 min                     20                              2160
        2 min                     30                              3240

So a BRT carries about the same at 17-18 buses per hour. 

Does that mean we shouldn't build BRT when the capacity would be lower? 


It means that BRT scales better than a general traffic lane. From the examples above, it's pretty clear that a bus every two minutes is feasible. Which means that a dedicated lane of BRT has a capacity of over 3000 persons per direction per hour, or about half again what a general traffic lane is.

Now, the really big, really nice BRT systems don't just have one BRT lane: They have two. Some have four (for local and express). Those are the places that really have a 'surface subway'. The capacity that arrangement provides must be huge. 

Light Rail
Wikipedia says LRT has a capacity of 220 per car. Assuming the calculations above apply, that gives us something like: 

Headway------- Traincars per hour-----------Capacity
       20 min                     3                                660  
       15 min                     4                                880
        5 min                     12                              2640

(I have serious doubts of UTA's ability to run a train more than once every 5 minutes...the system just is not designed for it)

But that's with only a single car per train. If they couple cars into trainsets (and SLC's long blocks permit up to four cars per trainset), the max theoretical capacity is quadrupled. 

Headway------- Traincars per hour-----------Capacity
       20 min                     3                                2640  
       15 min                     4                                3520
        5 min                     12                              10,560

Those are crazy numbers. That means on game days at the U (when every train is packed, and UTA is running trains every 5 minutes) TRAX is carrying about 5.5 freeway lanes worth of people.

Can you imagine the mess on I-15 without it?