Page 146 - 360.revista de Alta Velocidad - Nº 5

Grande, Zacarías. Blanco López, Marta. García Tamames, Alberto. Castillo, Enrique.

3.6 Safety correction of a speed limit signal

In this example the safety at a speed limit signal is improved. Initially an erroneous announcement
of a 120 km/h speed instead of 80 km/h causes the safety problem leading to an ENSI value of
0.145, which is extremely high (see Figure 15). This is due to the fact that the ASFAdig does not
cover the speed excesses. To correct this sittuation, an additional announcement of 80km/h
has been incorporated (see the blue shadowed plot in Figure 15) leading to an ENSI value of
-9,
3.52 x 10 which is satisfactory. In addition if the ASFA-dig is replaced by the ERTMS, the ENSI
-12
reduces further to a value of 1.58 x 10 , which is three orders of magnitude smaller (see the
green shadowed plot in Figure 15). This example illustrates the importance of speed limit signs,
which can be permanent or temporal. The fact that permanent signs are not covered by certain
ATP systems should be corrected to avoid accidents. In addition, the location of the sequences
of speed limit signs must be done very carefully.

Figure 15 Illustration of how the safety improves at a speed limit signal by correcting errors.
In Table 6 the circumstances under which the incidents at the speed limit signs are shown for
the cases speed-m, speed-b and speed-mb, respectively. It is interesting to see that the case
speed-m in which an erroneous announcement was located, the incident occurs, as expected,
at this speed, with no error in the driver decision at practically all cases. In case speed-b, the
speed reduces to 100 km/h and the driver decision remains correct in 86,318 % of the cases,
but erroneous in 13,676% of the cases. Finally, in case speed-mb we have again no error and a
speed of 100 km/h in almost all cases.

144 360.revista de alta velocidad