In several recent conversations involving Cambridge's developing master plan, I've heard references to an Alewife commuter rail stop as a possible solution for the highly car dependent Alewife area. I think it's a decent idea, especially when combined with a pedestrian overpass, but the current MBTA commuter rail operating practices make it unlikely to generate significant ridership. Fundamentally, it comes down the the concept of frequency, and what makes transit useful.
Frequency: The great decider
When it comes to what makes people ride transit, one of the most common perceptions is that the service provided, in terms of physical infrastructure, determines ridership. For example, in a situation where a light rail line replaced a bus, ridership can be assumed to rise. This perception is partially correct, in that a totally equal service provided with Bus Rapid Transit and full Rapid Transit will experience higher ridership with the steel wheel version. However, there is a far more important characteristic determining ridership, which is essential to understand the poor performance of commuter rail in the United States.
First: Commuter rail ridership
In order to make my argument, it's useful to look at ridership figures for the commuter rail, using the MBTA's invaluable Blue Book. In particular, lets look at Porter Square, which is an "Outside the core" station with both commuter rail and subway service, in addition to several bus routes. If the hardware behind the service determines ridership, we should reasonably expect to see the comfortable, speedy commuter rail in first, more than one thousand riders spread over multiple routes. However, in reality the opposite occurs. By far the most popular mode is the subway, followed in a distant second by buses. In an abysmal third comes the commuter rail, with an absurdly small 250 inbound boardings.
What's happening: Frequency
With these numbers in hand, it would be useful to find a new new deciding factor, one that would predict Subway->Bus->Commuter Rail at Porter, rather than Commuter Rail->Subway->Bus. It comes in the form of frequency. Instead of looking at speed, we need to look at time to access: how long one person needs to wait before transportation arrives, whether it's in the form of a train, bus or rickshaw. In addition, we should look at service consistency: the knowledge that at any time of day, that person could show up and expect transportation quickly. A subway is the ideal model of this, and commuter rail does heinously. When you show up at a Red Line station, you know a train is coming in a maximum of ten minutes (unless the MBTA has discovered a new way to break down, but I digress), and importantly, you know that will be true all day. In stark contrast, the commuter rail has no such guarantee. If you arrive in rush hour you may have to wait "only" twenty minutes, but if you arrive in the middle of the day, you may have to wait for up to two hours. If you need to get somewhere in a hurry, the commuter rail is exactly what you don't want: long waits that you cant predict, short of memorizing an entire timetable. This creates a situation where only 250 people decide to ride the commuter rail, where at least 12,000 choose to ride more frequent, yet less comfortable, modes.
What this means for Alewife:
So what does this mean for Alewife? The short answer is, don't expect great, or even good, ridership for any new commuter rail stops. The long answer also includes a hint for what we should do: increasing frequency, even without superior modes, produces disproportionate increases in ridership. Therefore, a better idea to increase transit ridership at Alewife is perhaps the most basic: better frequency, measured by the mean time between a bus departing a stop and the next one pulling in. For frequency, a great general rule can be described by who a given headway is useful for.
1 hour: Useful for almost no one
30 minutes: Useful for students and the poor
15 minutes: Useful for those without a car
10 minutes: Causes people to leave the car at home
5 minutes: Causes people to leave the car at the dealership, unsold
You'll notice the most dramatic effect occurs at five minutes, exactly the Red Line's average frequency.