BRT vs. Traffic Lane 2

A traffic engineer friend of mine was good enough to point out some issues with my earlier post. 

One flaw in your math is that 1900 is not what arterial streets carry.  That is the “ideal saturation flow rate” which means if there were green lights all the time and no other interference, you’d probably measure 1900.  From there you apply reduction factors. The biggest factor is the green-time factor, which may be .6 on arterials, and .35 or so for collectors.  So .6*1900 = 1100.  But in truth, a road like the Provo BRT corridor will be closer to 750 or 800 vphpl.  Then you have the occupancy factor, which at peak times might be 1.3 or 1.4.  So say 800*1.4 = 1100 people/hr/lane, if carried in cars.

Taking this into account, I'll set forth another set of scenarios:

First set is a full BRT with 90 person on it. This is a bit flattering to BRT, because it assumes that the BRT is 'full' all the time.  It was a simplifying assumption. But the ideal BRT would move more people than the ideal traffic lane. (2439 vs. 1596).

But if we make that more realistic, and assume that the bus is half full during the peak hour (perhaps generous, but plausible), the numbers are much less flattering to BRT. Even at max buses/hour, it moves about as many as an actual travel lane (1215 vs. 1330/1064). However, during rush hour, 30 buses/hour compares favorably: 1215 vs. 1045/836. Hence, during rush hour, a BRT simply carries more persons than any traffic lane, even at 50% full.

 However, dropping the number of buses per hour significantly undermines that advantage. As 5 minute headway (12 buses/hour), a half-full BRT only has a capacity of 486, less than half of that for traffic lane, even during rush hour. Each bus would need to be full (90 people) during rush hour, to equal the capacity of a traffic lane. 

For automobile travel, passengers per vehicle is the real wild-card. As the chart shows, an HOV lane, even minimally loaded (2 persons) at its worse (1520) carries more persons than all the BRT systems except the full loaded BRT at 3 or 4 minute headways. 

This suggests a system of on-arterial HOV lanes would be a more effective strategy than BRT. But that will be the topic for a later post. 

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? 

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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? 

No. 

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?

[1] http://www.metro-magazine.com/bus/news/719169/utah-transit-to-add-35-more-60-foot-new-flyer-xcelsiors

[2] https://www.nbrti.org/docs/pdf/EmX_%20Evaluation_09_508.pdf

BRT and Congestion

The principle of equilibrium assignment suggests that it is unlikely that congestion will change much on the corridor. If BRT successfully reduces automobile congestion on the corridor, travel will be faster in that corridor, and Down’s ‘triple convergence’[1] from alternate routes, times and modes will occur. In that context, the amount of congestion experienced by automobile drivers on the BRT corridor is unlikely to change significantly. However, from a system user perspective, the BRT may provide substantial benefits by actually reducing the amount of diversion (and out of direction travel) that is currently occurring. If this is so, it would be reasonable to expect a drop in volumes along the diversion corridors. It seems likely that the combination of ITS features and dedicated transit guideway will serve to increase the overall capacity of the roadway, and that a drop in traffic volumes on the diversion corridors is a reasonable hypothesis.

However, if congestion increases, a ‘triple divergence’ to alternate routes, times, and modes will occur. How much diversion occurs will depend on how attractive the alternatives are. Assuming no significant addition in roadway capacity on alternate corridors, diversion to alternate routes will result in a slight worsening in overall congestion. Diversion to alternate times will make the ‘peak hour’ longer (AKA ‘peak spreading’). Diversion to other modes may or may not reduce 

congestion.

Buses in general traffic lanes reduce capacity and increase congestion, a phenomenon well asserted both by the literature and by experience. The core principle of making transit ‘rapid’ is removing transit vehicles from general traffic lanes. This serves to both remove the effect of their operations on automobile traffic, and remove the effect of automobile congestion on transit vehicles.

As a thought experiment, assume the BRT is very attractive (in terms of time or cost), and attracts a large number of riders. This reduces automobile congestion along the alignment, making it faster. Drivers diverge from other modes and other routes, and the corridor becomes congested again. But only for automobiles--due to exclusive guideway, the BRT is less affected, and remains an attractive alternative. For drivers on the BRT corridor, there is no net benefit. For transportation system users, there are two classes of beneficiaries: BRT riders, and drivers on the diversion corridors.

A caveat to the benefits to drivers: The benefits to drivers on the alternate routes is going to get ‘lost in the noise’. They will be dispersed over a large number of roads, and reflected in small changes in the duration of peak periods, or in minor traffic volumes in a large number of roads. Provo-Orem is a rapidly growing metropolitan area, with substantial development taking place both north and south of the study area. Any minor advantage from the BRT to drivers will be rapidly eroded by additional land use changes.

A caveat to the benefits for riders: ‘rapid transit’ implies exclusive guideway; most BRT systems are only ‘semi-rapid’. While provided with transit signal priority, time separation (at intersections) provides a reasonable analogue to rapid transit conditions. However, the Provo-Orem BRT has only 51% exclusive guideway. Where the BRT lacks dedicated guideway, it will be exposed to the effects of congestion. In ideal circumstances, this guideway will be placed in the most effective location; where congestion is most intense. Congestion also tends to be greatest near intersections. Thus, roadways tend to be widest at intersections, where the road shoulder is used to provide turn lanes. Many worthwhile BRT projects have been subjected to the ‘death of a thousand cuts’; minor sacrifices made in the name of preserving automobile capacity (or worse:maintaining on-street parking).

However, given the number of routes that the also service parts of the BRT corridor[2], it is unlikely that all of the delay induced by local buses will be eliminated. In the context, it seems likely that the corridor will stay at a very similar level of congestion. 

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[1] https://escholarship.org/uc/item/3sh9003x#page-4

[2] http://www.rideuta.com/-/media/Files/System-Maps/2016/Utah-County-System-Map.ashx