UPDATE: BeyondDC informs me by e-mail that, although you wouldn’t know it, this vote implies at the very least that Maryland has a strong preference for light rail, as that’s what the state asked the TPB to model the pollution effects of. If light rail is close to a done deal, read this post as an argument that transit advocates should pay more attention to BRT and not always preface discussion of it with “it’s certainly not rail, but it’s an important part of a transit system.” If it’s not—say, if Maryland decides that their budget is just hurting too much and they want to give BRT a second look—so much the better.

As I’ve already mentioned, the Purple Line is rolling forward, having received unanimous approval from the National Capital Region Transportation Planning Board. It is now awaiting Gov. O’Malley’s signature.

There has not yet been a final decision, however, on whether the Purple Line will be light rail (LRT) or bus rapid transit (BRT). This is a critical decision that will apparently be made by the end of the summer.

All the momentum seems to be pushing for light rail. Both the Montgomery and Prince George’s County councils and executives have endorsed light rail, as has the Washington Post. There seem to be two major sets of arguments against BRT.

The first and most prominent is that buses are a second-class transit alternative. This sentiment is perfectly expressed by Del. Tawanna P. Gaines (D-Prince George’s), chair of the House subcommittee on transportation appropriations, who is quoted in the Washington Post saying that “In Prince George’s, our public bus system has fallen short. We don’t have confidence in a new bus system. I think most people think buses aren’t as good.”

The second argument against BRT is that it has less capacity than LRT. You can see that argument in the Post’s editorial and in this post from Greater Greater Washington (read the comments, too). Lower capacity, these opponents argue, means fewer people out of their cars.

On the other hand, the strongest argument in favor of BRT comes from a report by the World Resources Institute, an environmental think tank and the parent organization of this blog. This detailed analysis compares BRT and LRT on two metrics, cost-effectiveness and greenhouse gas emissions.

Let’s start with cost-effectiveness. WRI defines cost-effectiveness as dollars per hour of user benefit, with costs including both capital and operating expenses and benefit including variables such as travel time reduced and the number of users. They have two fantastic graphs that show their findings. The first assumes that the state’s ridership forecasts are accurate; the second assumes that they are overly optimistic.

As you can see, you get significantly more bang for your buck with BRT than with LRT. This is because while capacity is marginally lower, costs are significantly lower (see the report for more graphs!).

Second, WRI finds that BRT will reduce greenhouse gas emissions while LRT will actually increase them. Here’s another graph:

The report goes on to argue that both the BRT and LRT emissions should be lower than these numbers (for BRT, ridership should be higher than in the data this graph uses; for LRT, the system will be drawing from a sub-regional power grid that is cleaner than the Maryland average, which this graph uses). The difference, though, remains quite striking.

So which side has the better argument? It depends.

Specifically, it depends on how closely you think that BRT can substitute for light rail or more accurately, the extent to which Maryland will make BRT a close substitute for light rail.

The low-investment BRT, for example, “is mostly at-grade and operates in shared use lanes on existing roadways,” according to the State of Maryland’s guide to the Purple Line. That’s simply not as good as light rail. It will feel like Metrobus, with fewer stops.

On the other hand, the high-investment BRT travels almost exactly the same route as the high-investment LRT. There are two big differences here. The first is that it looks like a bus, not a train. The second is that it has an internal engine instead of taking power from overhead wires. That’s about it, though. Concerns about it being a second-class mode of transit can basically be ignored. Throw in some good design to make the buses look sleek and attractive and you’re set.

A high-investment (or medium-investment, mostly) BRT would also assuage my personal concern about BRT, which is that it have the obvious permanence of light rail in order to spur transit-oriented development. With investment, a BRT Purple Line would.

One final point. Cost-effectiveness is currently the number one factor in determining which transit projects do and do not receive federal funding under the New Starts program. While this could change if Rep. Oberstar passes his new transportation bill, under current law, a BRT Purple Line has a much better chance of getting federal dollars. That’s another big point in BRT’s favor.

To sum up an overly long post, if done correctly there is no reason for a bus to be seen as second-class nor for there to be less demand to ride the BRT nor for the BRT option to have lower capacity. When the high-investment BRT option is still lower-cost than the low-investment LRT option and reduces, rather than increases, greenhouse gas emissions, this seems like a no-brainer. That said, cheaping out on the BRT seems like a mistake to be completely avoided and building a light rail Purple Line would obviously be a major victory for sustainable transportation advocates, period. In other words, bad BRT,  fine; light rail, really great; good BRT, even greater. Good BRT is, in the ideal world, the goal.