From: Peter B. Ladkin (ladkin@xxxxxx)
Date: Fri 24 Dec 2004 - 08:51:08 GMT
Andrew Shore questions how and why speed limits are set. I don't propose to get into a discussion of whether and how road traffic engineers working for a democratic government represent a choice by society. But I think it is important to understand some engineering and behavioral reasons for setting speed limits. Engineering reasons are environment, physics, and planning. Environment. If obstacles are likely to appear in your path, if there are corners, if there are places beyond which a driver cannot see, if there is multiple use (pedestrians, cyclists, including children) and so on, then car drivers must be able to react and stop before an obstacle which appears. Physics. Newton's laws of motion. Coefficient of friction. Manoeuvrability. Planning. Signs and warning devices, curves, turn-outs all have to be planned assuming a given (maximum) traffic speed. All of these put constraints on driving behavior which are not understood or appreciated by the majority of drivers. To illustrate this point, consider the following. Tell me the stopping distance of your car under heavy but normal braking at 35mph in the dry. Now again in the wet. Most people can't. (And yes, I have calculated and memorised the theoretical stopping distances of my bicycles. But I was still surprised yesterday in moderately heavy rain; I hadn't taken into account for this particular bike - I have a few - the time it takes for the blocks to wipe the rims under heavily-wet conditions.) But traffic engineers know such figures when they are devising clearways for automobiles and other road users. They must devise these under certain assumptions, such as a maximum speed of traffic at certain points, and these assumptions must be codified into a speed limit in order to ensure their continued validity and thus the adequate safety of the construction. We assume those traffic engineers perform an appropriate analysis of the environment and the physics while performing their planning, and we may assume that they spent more time doing so than the few seconds which a car driver who is using that facility typically has in which to assess the situation. Now social reasons. Mark Bowell has indicated how individual decision making by drivers on what is appropriate or not can lead to interactive conflict. So leaving it up to individual drivers to determine what is adequately safe or not in given circumstances is not a modus operandi for which will ensure adequately safe traffic operation. There is a fifth factor, also a social reason, why it is necessary to adhere to speed limits. Road traffic is a social situation in which there are many agents. We must make assumptions about how other road users are going to behave. Car drivers passing me within the 1.5m-2m clearance that they are supposed to leave are proceeding under the assumption that I will not wobble left into their vehicle, and so far they have been correct in that assumption. One very important cognitive component to decision making, especially in towns, is to judge the time in which oncoming traffic will reach me, and to judge general manoeuvring capabilities of others. It is necessary to set a speed limit in order for all road users to make cognitive assumptions about those parameters that will enable accurate decision making: when I see a car *there*, I know I can cross without hurry and in adequate time to be seen myself. When much closer, then not. There may be no "right" limit according to this criterion: 30kph may work, also 40kph, even 50kph. But there has to be one. And people travelling faster than this limit are violating the assumptions that others are entitled to make about their behavior; they are ipso facto creating a situation which is less safe than planned or expected. One example from a few years back. When my son was tiny, I hauled him about in a trailer behind my recumbent. The total vehicle was long, some 4m or so. And we accelerated slowly (braking was good, because of the dynamics of recumbents). Although we were long, slow and wide, I suspect it was actually a lot less risky than travelling about on my own, because were were very yellow and comparatively large, so could be seen well under all conditions, and represented an object about which others had to manoeuvre. There is also a special taboo about endangering very young children. I cannot recall anyone passing too close in three+ years of using it. However, there was one major incident. There is a particular intersection with a middling-major road which we used to traverse regularly. Since traffic was parked at the side of that road, line of sight was impaired. But one gets judges the distance one must be able to see, and the time one has to cross, even allowing for those who might be travelling faster than the 50kph limit. Once, I performed the usual stop-and-carefully-look, saw no traffic at all, proceeded to cross. I was about two meters into traffic when I suddenly saw, and heard, a car travelling towards us at what must have been 80kph or so, well over the limit of 50kph and much faster than any traffic I had ever seen there. Brakes went on; I stopped in about another meter. But there I was, some three meters into the roadway with Simon behind me, with no further manoeuvring possibility. The car swerved into the opposite lane (there was no other traffic in sight). It was full of yelling youths. They had violated the assumptions under which I made my decisions. Were this a road in which such things frequently happened, then I would have been in the habit of using the intersection differently; in fact, I would have used a different route some 40m to the right, which I started to do after this incident, which has extended line of sight. If I had been in a car, I would likely have accelerated faster, taken longer to stop (constant reaction time at slightly faster speed; slightly lengthened stopping distance) and there could well have been a collision, since there would have been less room for the other to manoeuvre away. And the alternative route, which is a pathway, would not have been available to me in the future. Speed limits set assumptions under which roads can be effectively jointly used. There is no way of judging a speed "safe" or not from an individual perspective if it is higher than the set limit unless one is a very thorough socio/anthropologist of precisely that microenvironment. To summarise: because of the constraints of environment, physics, and planning assumptions under which roadways are engineered, judgments by individual drivers, no matter how smart or aware they may be, on "safe" speeds are necessarily unreliable. Further, Mark Bowell's example shows that coordination of many road users does not follow from individual optimisations, but must be superimposed through general constraints (such as speed limits). Finally, road users develop practical expectations of the behavior of others, which is constrained by a maximum expected speed, and violations of those practical expectations invalidate individual decision making and thereby increase risk in many situations. Those are five good reasons why it cannot be left to individuals, even smart, experienced individuals, to judge appropriate traffic speeds as they pass through. On another matter raised by my example: there is quite a lot to be said from the safety point of view for raising the driving age to 25. And for introducing a year or two of compulsory civil service, especially in ambulances and clean-up crews for males, for all citizens of ages 18-20 or so. PBL -- Peter B. Ladkin, Professor of Computer Networks and Distributed Systems, Faculty of Technology, University of Bielefeld, 33594 Bielefeld, Germany Tel+Msg +49 (0)521 880 7319 www.rvs.uni-bielefeld.de