Whenever you hear someone cite the travel time of a proposed transit line, your first reaction should always be: “Yes, but at what frequency?” Often, that fact is missing from these soundbites.

Here’s a nice example from a 2010 Transport Politic report. It refers to the Gold Line Foothills extension to the Los Angeles light rail system (then proposed, now under construction), which, when finished, will extend the line to northern suburb of Montclair.

The commute times from the end of the [proposed Gold] line will be a serious problem: once the second phase is built to Montclair, downtown will be a full 75 minutes away, making daily commutes difficult to envision for many people. Even in traffic, that trip takes a total of 70 minutes by car.

Nevertheless, getting people from the San Gabriel Valley into downtown may not be the major goal of the project. [The existing] Metrolink Commuter Rail can cover the distance between Montclair and Los Angeles in an hour along the San Bernardino Line, though that link is near carrying capacity.

If all you care about is commutes during the peak hour, then and only then do you have all the information you need. In many North American cities, it’s only during the peak commute period, and in the peak direction, that you can expect anything called “commuter rail” to serve you anywhere near your desired time of travel.

But if you care about supporting denser and more urban redevelopment, or helping people own fewer cars, or being relevant to the lives of service workers, or serving students who travel all day… If you care about any of those things, then you’re missing a crucial fact: the proposed all-day frequency.

Frequency is best described by headway, the elapsed time between consecutive trips on a line. The headway is also the maximum waiting time, and half of it is the average waiting time. Add those to the much-advertised travel time (technically called in-vehicle travel time or IVTT) and you have a sense of how long a real-life trip will be. Those are the realities that will ultimately drive ridership.

The 2010 San Bernardino line timetable. You lucky people. Image: Metrolink.

This makes all the difference in the comparison above. The Gold Line can be expected to run every 15 minutes or better all day, which is pretty much the industry standard for light rail, though some agencies run less service during the late evening. Meanwhile, the Metrolink commuter rail line in this comparison runs every 60-180 minutes midday.

To get a quick sense of how useful a service will actually be to everyone, not just peak commuters, I always check a sample trip around 12:00 midday. The commuter rail headway is 120 minutes at that hour, so for an average trip time I take half of that and add it to the advertised in-vehicle travel time:

Average travel time = Headway / 2 plus in-vehicle travel time

Commuter Rail: (120 minutes)/2 + 55 minutes = 115 minutes.

Proposed LRT: (15 minutes)/2 + 75 minutes = 83 minutes.

Big difference. And that’s just the average. The maximum travel time is …

Maximum travel time = Headway plus in-vehicle travel time

Commuter Rail: 120 minutes + 55 minutes = 175 minutes.

Proposed LRT: 15 minutes + 75 minutes = 90 minutes.

North American commuter rail lines are typically very, very infrequent midday, if they run at all. The Metrolink line in question here (the San Bernardino line) is hourly in the mid-afternoon, but has a two-hour gap mid-day (which affects the above calculation) and a gap of almost three hours in the morning.

These gaps happen even on a line that is “near carrying capacity”: that’s because Metrolink shares its tracks with other uses, including freight and the occasional Amtrak train, and is also constrained by single-track segments. All this means that it tends to flow a lot of service one way during the peak, but has trouble serving reverse-peak or midday trips.

The real lesson here, of course, is that when you hear someone cite a commute-hour in-vehicle travel time for a proposed service, you should ask: “Is the commute all that matters?” Journalists often think so. 

But if you’re taking a longer view, and wanting a project to

(a) be part of an integrated network, and

(b) be useful all day so that

(c) it can trigger redevelopment and

(d) enable people to own fewer cars

…then the commute-hour travel time should be worthless to you.

Ask instead about the travel time around 12:00, and always remind them that in the real world, travel time includes wait time. So what’s the frequency?

Jarrett Walker is an international consultant in public transit network design and policy, based in Portland, Oregon. He is also the author of  “Human Transit: How clearer thinking about public transit can enrich our communities and our lives”, which you can buy here.

This article was originally written for his blog in 2010, and is reposted here with permission.