One of this blog's recurrent themes is that we need to notice when people are thinking about transit as though it worked just like roads and cars. Our transportation bureaucracies are full of people who've been trained to understand traffic, and who sometimes struggle to extend that mental framework to transit. One of the most important American "bibles" on public transit, the Transit Capacity and Quality of Service Manual, was explicitly designed to imitate the structure and style of the AASHTO Highway Capacity Manual, because it saw traffic experts as one of its key audiences.
In the general public, too, it's common to encounter well-intentioned people who unconsciously think like motorists even as they try to talk about transit; one sign of this is when people talk about vehicle speed as though it were much more important than frequency. For cars it is. For most urban transit it isn't, with the exception of the most rigidly scheduled commutes.
Along these lines, consulting engineer Steven Kolarz wrote me to ask about this famous diagram from the AASHTO Green Book, explaining the tradeoff between access and movement as highway planners understand these terms:
An interstate highway has great mobility (manifested as high speeds), but poor accessibility (wide spacing in interchanges). Meanwhile local streets have the opposite - poor mobility (low speeds), but high accessibility (numerous intersections, curb cuts, etc). Perhaps it is time we adapt this idea more explicitly to transit. You already briefly touched on this idea in the controversial, but well needed, streetcar posting. Though true that the exact implementation is a transit technology can be very fluid, there are overarching features and implementations that can help dictate what is appropriate and inappropriate. Specifically, I am thinking of inappropriate as an 89 mi. High Speed Rail line, and a 64 mi. Light Rail line.
Yes, that's what I'm up to when I talk about the balance between distance (which requires speed) and stop spacing (which causes delay) in posts such as this one. (I would clarify, though, that there's not necessarily anything wrong with a 64 mile light rail line, as long as it's fast in the middle where the most people are on it. But most long light rail lines in North America are the opposite; they have downtown in the middle and go through downtown on-street, making that the slowest segment.)
The speed vs stop spacing issue in transit is reasonably analogous to this speed/access distincion in highways. In transit, travel time is determined mostly by what can get in your way, and by how often, and for how long, you have to stop. You could even make an analogy between a typical local bus stop, where the bus stops only if requested to, and a driveway access to an arterial, which causes delay to through traffic only if it's being used. In this analogy, a major traffic signal that affects most traffic would be comparable to a rapid transit stop where every trip stops because there's always someone getting on or off.
This tradeoff between speed and stops in transit determines some of the most fundamental categories of transit service: local vs. rapid vs. express as I defined them here. This is an area where transit/traffic analogies work reasonably well.
The real problem for leading highway thinkers across the conceptual gap into the transit world is frequency, a concept of paramount importance in transit that has no analogue in motoring, apart from the small waits required by signal cycles. This is such a persistent conceptual barrier between motorists and transit people that I've started coining vulgar alliterative slogans such as "Frequency is freedom!" Because it really is.
Finally, when interacting with highway engineering, it's crucial to notice that the highway engineering meaning of the terms access (or accessibility) and mobility (or movement) is quite different to the way those words are used in sustainable transport planning and urbanist thinking. The terms access and movement in the email and diagram above refer exclusively to the tradeoff as it affects vehicles. If everything that matters is a vehicle, then access and mobility do come into conflict, and the different standard road types, from freeway to cul-de-sac, represent different points of balance between them, as the diagram indicates.
But when sustainable transport people talk about mobility and access, they're extending the concept to apply to all humans rather than vehicles, which makes the issue vastly more complex and multidimensional. They're also transforming access into something more purposeful, and assigning it a moral weight that it doesn't have in the highway world. In the sustainable transport formulation, as I summarized here based on Todd Litman's excellent summary, mobility is how far you can go, while access is how many desirable things you can do. You can improve your access, but not your mobility, by moving closer to work, or moving in with your romantic partner. In this urbanist formulation, access can be improved through by putting desirable things closer together -- a process that we all consider when we decide where to live, and which urban designers do in the aggregate when they design or redesign communities in response to the demands that our individual decisions have generated. For that reason, access, in Litman and similar urbanist thinking, has taken on a moral superiority over mobility which it doesn't have in highway thinking.
So be careful with mobility or movement and access or accessibility. Urbanists and sustainable transport people mean one thing. Highway engineers mean something much simpler -- so simple in fact that you can make a diagram of it.