In any profession’s quest to be more rigorous, it minimises the uncertainty in its knowledge. We have in transport planning instead aimed to minimise the perception of uncertainty. But the uncertainty is still there. It comes in a few flavours.
Dark Travel and Ghost Travel
We might think about how much travel is measured.
Applying the Rumsfeld Typology:
Travel that occurs, and observers know occurs -- Correctly Measured Travel
Travel that occurs, but which observers don’t know about (false negatives or Type II errors) — Dark Travel1
Travel observers think occurs, but doesn’t actually happen (false positives or Type I errors) — Ghost Travel
Travel that observers correctly assess didn’t happen — Correctly Measured Non-Travel
Dark Travel
For automobiles and other motorised modes, we have smart phones, odometer readings, fuel consumption, traffic counts, toll readers, and passenger counts, among other sources documenting this reasonably well in the aggregate. But even so, not every trip is systematically documented, and there may be offsetting errors, and thus there is Dark Travel.
For active transport modes, there are few detectors, some people may be tracked by cell phones, and travel surveys can be used to make estimates, but by and large this is systematically undocumented, and almost certainly systematically underestimated in official statistics relying on measurements without corrections. Children’s travel and many other “niches” that don’t fit into the standard paradigm are large parts of this. Many sources, like home-based travel surveys, underestimate truck and business travel.
Ghost Travel
The counterpart to Dark Travel is Ghost Travel. This is travel we measured and think we know, but doesn’t actually take place. The classic example is the inflatable dummy in the car using an HOV lane. The occupancy of the car is assumed to be 2 or 3 passengers, but is only 1. I have seen other examples, people gaming the Opal Farecard system, by tapping in multiple Farecards simultaneously for a short (and low cost) trip, to more quickly reach the weekly all travel after 8 trips per week is free, presumably to use those fare cards for longer trips by family members. There are other sources. Odometer and fuel readings give estimates, which can bias the total amount of travel if some of that fuel is used for non-travel purposes. Travel surveys may report trips being made that weren’t actually. A GPS based survey analysis may break a single trip into multiple trips, inflating distance (if not time). GPS drift may increase total distance traveled. I am sure there are other sources.
Almost by definition, we cannot have accurate estimates of Dark Travel and Ghost Travel, because they are based on the discrepancies of measurements, and there is no fully complete source of truth. At best we can triangulate from sources of misinformation. Thus the amount of both Dark and Ghost Travel is unknown. Since we are talking about it though, they are both known unknowns.
Dark Access and Ghost Access
Relatedly, let’s look at our choices, not just our decisions.
Access is the ease of reaching destinations.
But what is “ease” and what are “destinations”
The study of observed and perceived access reveals significant differences between the two. Our research has shown that people misperceive both travel time and opportunities. They may make Type I errors (false positives) or Type II errors (false negatives). That is, they may think they have access when they don’t, but more likely, because people tend to overestimate travel times, they may think they don’t have access when they do.
Access people have and know they have -- Correctly Perceived Access
Access people don’t know they have (false negatives) — Dark Access
Access people think they have but don’t (false positives) — Ghost Access
Access people correctly assess they don’t have — Correctly Perceived Lack of Access
For every individual there will be both Dark Access, destinations they could reach, but don’t realise it, and Ghost Access, destinations they can’t reach as quickly as they thought.
Dark Access
Dark Access can arise from many causes.
First, things may be faster than we expect (i.e. we overestimate the time and cost of reaching the place). This is especially true for modes people are less familiar with. People estimate the travel time for unused modes as higher than we measure it to be. This is in part why they may not use that other mode.
Second, there may be more opportunities than expected. However, our evidence is that people also overestimated the number of jobs in the CBD (though were more accurate for an estimate of the number of jobs in the metropolitan region as a whole). So there is probably some net bias to overestimating job opportunities, the more so for transit users.
How well people estimate times to non-work activities, or their number, is unknown.
So how that nets out depends on the amount of overestimation.
Ghost Access
People often underestimate travel time (time flies when you are having fun), so think they can reach some places faster than it actually takes. This tends to be on trips with low mental transaction costs (e.g. as a car passenger, or a driver of a car on a freeway with no congestion and no turns).
While GPS units and other information sources can help align perceived and actual access (and thus minimise Dark and Ghost Access), the extent to which people rely on that is also unknown. So the amount of both Dark and Ghost Access is unknown. Because we are talking about it though, like Dark and Ghost Travel, they are not merely unknowns, they are both known unknowns, things we now know that we don’t know.
This term has other connotations, not the least of which is travel at night, or travel to mysterious places. But the term is just so cool.
The travel-related thinking is one of the observer (who is supposed to be qualified and have the knowledge to interpret the data keeping in mind the known unknowns).
The access-related thinking (as presented here) is one of the user.
The perception of access from the user experience point of view, as I would think of it, must be entirely different from one of a researcher: people would think about their access in relation to their immediate problems to solve, and until they are intentionally looking for pathways to get to a new (potential or required) destination, they would hardly ever consider whether they can do so (and, if yes, how exactly).