Effectiveness of variable feedback signs for speeds on secondary rural roads in the area of Bari (Italy)

While driving along most of the secondary road network (and not only), it is clearly perceivable how most of the vehicles move faster, and sometimes far faster, than the speed limits that are there enforced and signalled. Another clear sensation is that the speed limits themselves are too restrictive in comparison to the operative speed which is consistent with the geometric characteristics of the road. The observed operative speeds, in fact, are, quite often, far higher than the enforced speed limits. It is widely accepted that the operative speed of a specific road section is strongly dependent not only on the relevant speed limits but also on other explicative variables such: number of access, activity on road sides, width of lanes, width of shoulders, curve radius…. (K. Fitzpatrick et al., 2003). These variables decisively influence the driving behaviour and often lead to a huge gap between the enforced speed limit and the operative speed. In Italy, as all over the world, the enforced speed limits are, nearly always, lower than the operative speed. Repeatedly, then, has been raised the question of adopting speed limits which turn out to approximate the operative speed as much as possible. On the other hand, as for speed, it is not sensible to establish a unique criterion on which a road can be designed and enforcing the relevant limits wouldn’t be sensible as well. In many cases, in fact, even though the environmental and geometrical characteristics could admit a high operative speed, some hidden or unpredictable dangers might exist and they must be considered as well. That is the reason why some extremely restrictive speed limits are often observed, in comparison to the potential operative speed of a specific road section. So, in rural roads it is quite easy to find the speed limit set to 50 km/h (the ordinary limit would be set to 90 km/h) even though that road section could admit a far higher operative speed. Evidently, the enforcement of such limits is explainable with the high probability of meeting farm tractors or other extremely slow agricultural vehicles. From there comes the need to narrow the existing gap between moving vehicles, as it is widely proven that one of the major source of road accidents is an excessive speed gap between moving vehicles (R. Mussa, 2004). This problem clearly cannot be solved with the simple imposition of restrictive speed limits, but it is linked to other and more complex issues related to the management and the design of roads. Moreover, the problem is that the frequent misuse of such limits leads drivers to a wrong perception of the actual danger that is behind such enforcement. Such limits, then, are often totally disregarded as it’ was observed in the specific situation described in this study. So, when there’s a real need to lower speed in order to reduce the risk of a road accident, it is necessary to improve the ordinary speed limit signals with other speed reducing devices such humps, rumble strips, narrowing road width, etc.. In this way, a good result is given even if only in the immediate surrounding of the site where such devices are installed (M. Pau and S. Angius, 2001; R Elvik and T. Vaa, 2004). In addition to such “mechanical” devices, there are other less invasive ones like automatic queue warning signs, individual feedback signs for headway, individual feedback signs for speeds, etc. Such signals basically warn drivers against their hazardous behaviour. But, on the other hand, if it’s been possible to observe encouraging correlations between the use of such devices and the reduction of road accidents, it is equally true that, generally, the driver (in spite of having been warned) tends to maintain his behaviour considering himself in a safe position (R Elvik and T. Vaa, 2004). Truly, the only real way of inducing drivers to lower their speed, effectively and constantly, along a specific road section, is based on warning them of the real probability of being fined, which is achievable either with a remote surveillance system or with the physical presence of policemen (C.A. Holland and M.T. Conner, 1996). At the Polytechnic of Bari an experimental investigation has recently carried out, whose aim is to study the drivers’ speed behaviour along the secondary extra-urban road network of the hinterland of Bari. Among the global study, a first investigation has been carried out to study drivers’ reactions in presence of individual feedback signs for speeds.