People and their Paths 2: Why aren’t people taking the shortest path?
October 15, 2013
Recall in yesterday’s post we considered Wardrop’s User Equilibrium Principle, which says: the journey times on all routes actually used are equal and less than those which would be experienced by a single vehicle on any unused route. We then asked if people were taking the shortest travel time path, and came to conclude no.
This leads us to the next question:
Why aren’t people taking the shortest path?
Here are a few conjectures:
- Selflessness: The principle assumes that people are selfish, but perhaps they are selfless. We assume they aim to minimize their own travel time rather than society’s. We will come back to this point in Episode 5, but for now, let it suffice to say that people cannot know what decision will minimize society’s travel time, because of computational and informational issues, discussed below. Perhaps if they had that information, they might selflessly choose a different route. In the absence of that information, they are, at best, left guessing whether what they are doing is best for everyone else, even if at some self-sacrifice. (This assumes they are still making the trip at that time. In general, if there is congestion, it would be better for everyone else from a travel time perspective to avoid the trip altogether).
- Rationality: The principle assumes that people are rational, but maybe people aren’t rational, or at least not rational all the time. In one sense that is of course true, people react emotionally and intuitively, employing what Nobel Prize winner Daniel Kahneman calls System 1 in Thinking Fast and Slow 1, based on heuristic rules. They don’t have time for rational assessment. In another sense, for a repeated decision like commuting back and forth to work daily, it costs a significant amount of travel time, a scarce resource, to systematically behave irrationally. We thus assume people are behaving rationally (engaging Kahneman’s System 2) when they can.The idea of Bounded Rationality, developed by Herbert Simon, also a Nobel Prize winner, has been applied to route choice problems by many researchers, including my Master’s Advisor, University of Maryland Prof. Gang-len Chang in his dissertation work with Prof. Hani Mahmassani at the University of Texas. We can build models with bounded rationality assuming or estimating the bounds to this rationality due to information, cognitive limits, and time available to make a decision. We discuss some of the factors described below.
- Perception: It might be that people think they have the shortest travel time on their route, but they misperceived the travel time on the network.There are perception or cognition limits. On a 24 minute trip, are you going to know what the travel time is to the nearest 30 seconds or minute? I would because I’m a transportation geek, but most people aren’t going to measure their time that precisely. When you look at how people report travel times in surveys, they typically round to 5 minutes and sometimes they round to the nearest 15 minutes. If people are only dealing with time perception in 5 or 15 minute chunks, saving a minute or two isn’t going to show up on their radar as something that is important to them.There are many other aspects to the perception of travel time, which we will discuss more in episode 4.
- Computation: Sadly (for us modelers), people are not computers. They cannot accurately add travel times across different road segments, they can’t systematically compare the travel times over alternative routes even if they had a complete data set.
- Information: Not only are people not computers, they are not GPS systems either. People don’t have complete maps of the network; people often have good mental maps of the local street network around where they live and a little bit around where they work and where they travel frequently, but if they live far from where they work, they tend not to know the detailed network in-between. There are limits to people’s ability to navigate. People’s cognitive or mental maps are far from complete. They only have the experience of the routes they have actually used. They can test other routes to gain experiences but they don’t have those innately.
- Valuation: Maybe people are minimizing the weighted sum of travel time, where time spent in different conditions is valued differently. We know, for instance, from the transit literature that time spent waiting for a bus is much more onerous than time while on-board a vehicle in motion making progress towards its destination, especially if the arrival time of the bus is uncertain. In tomorrow’s post we will discuss valuation of travel time under various conditions.
- Objective: We assume that people care about minimizing only travel time. It might be that people are rational but they care about things besides or in addition to travel time.We have evidence from other transportation choices that people aren’t minimizing travel time. When you choose a place to live, you are not choosing to minimize your commute time to work. In fact there have been studies that have considered a hypothetical relocation of everybody’s place of residence in order to be in a house that was equivalent to the structure in which they currently reside, but was as close to their work place as possible (given everyone else was similarly moved), average commutes fell from about 24 minutes to 8 or 10 minutes. There is a significant amount of “excess travel” from a strict travel-time minimizing perspective. There are lots of reasons for excess travel, but the most obvious is that it is not excess from the point of view from people who are making it. They’re making home location decisions for a variety of reasons; the journey to work isn’t the only thing in their mind. (Travel time must be a consideration though, otherwise cities wouldn’t exist). It might be when choosing where to reside people might underestimate the amount of time that will be spent traveling, and probably underestimate the pain associated with long commutes, and are thus unhappier than expected. Stutzer and Frey call this phenomenon The Commuting Paradox. A major source of time estimation error arises because most people search for homes on the weekend, but tend to commute on weekdays.Some candidate factors for route choice are given below:
- Search cost: How long does it take to figure out what the travel time is on alternative routes? Are you willing to spend ten minutes exploring the network in order to save 30 seconds of travel time every day for the rest of your career? Rationally it might be worth doing so, the payback is only 20 days. People often will discount the possibility of saving time, worrying that this short-cut will actually be longer, or maybe they’re afraid of getting lost. Fear of the unfamiliar is a major deterrent to exploration.
- Route quality: Many factors describe the quality or condition of a route and its environment. Is it potholed or newly paved? Does it run through a pleasant or unpleasant neighborhood? We have evidence that some people prefer a longer route if it’s an attractive boulevard or parkway rather than drive through a freeway trench.
- Reliability: The likelihood of arriving on time, and not just the expected travel time, affects willingness to select a route. There is the old parable of the man who drowned in an average of one inch of water. Similarly, it might not matter to me that the average travel time is 20 minutes if one day a week (but never knowing in advance which day) I can expect a travel time of 60 minutes. I don’t want to leave 40 minutes earlier to avoid the occasional bad outcome. I might be willing to take a slower but more reliable route. I might even have a mixed strategy, or portfolio, combining different routes in order to achieve a personally satisfactory trade-off between expected time and reliability. In practice this means some people might take surface streets, which are generally slower, but more reliable, instead of freeways, which are faster, but subject to more catastrophic breakdowns of traffic flow.
- Pleasurability of travel: Maybe people are rational, but they like traveling a little bit more than being at work or home, and so choose longer routes to prolong the experience.Many people want to commute; Redmond and Mokhtarian find there’s a positive value to some amount of commuting, that the preferred commute length is not typically zero. However, it appears that many commutes are longer the desired amount. However for some people, the longer route, which provides some psychological buffer between the stresses of work and the stresses of home, is desired.
The next two posts will consider issues of valuation and perception in more detail.
1. The terminology “System 1″ and “System 2″ apparently derives from Stanovich, K E.; West, R F. (2000). “Individual difference in reasoning: implications for the rationality debate?“. Behavioural and Brain Sciences 23: 645–726. The idea itself is much older.↩