Railway Signalling & Interlocking

13 Level Crossings 372 sary sighting distance from the sighting point A to the stopping point, which is usually at the St. Andrew's cross, can be calculated as follows: l t v l A r v b = + · and the complete clearing length as follows: l t v l l l C r v b lc v = + + + · with: lb : braking distance of the road vehicle (speed-dependent) llc : length from the stop position to the end of the conflicting area of the level crossing lv : length of the road vehicle tr : reaction time of driver and vehicle vv : speed of the road vehicle Accordingly, the clearing time can be calculated as follows: t t l l l v C r b lc v v = + + + The minimum approach time to avoid conflict is: t t S a C = + with S: safety margin [s] The approach length lB of the train is therefore as follows: l v t v t l l l v S B t a t r b lc v v = ⋅ = ⋅ + + + + ⎛ ⎝⎜ ⎞ ⎠⎟ with: vt : speed of the train Typical value ranges of variables are shown in table 13.2. Table 13.2: Value ranges of variables Variable Typical value range lb 5 to 100 m (depending on vehicle speed and brake deceleration) llc 5 to 20 m (depending on number of tracks and crossing angle) lv up to 25 m (depending on national upper limit for vehicle length) tr 1 to 3 s vv 1 to 30 m/s (depending on general or local speed restriction) S 2 to 5 s In some national cases or where special regulations apply, stopping is obligatory even if no train is approaching. In this case, tr and lb can be set zero, which means that the sighting point A is the stopping point (the position of the St. Andrew’s cross). In this case the clearing time tc must be higher because the acceleration of the road vehicle (starting up) must be considered. Therefore the approach length lB can also be longer. For a real level crossing, particularly where stopping in front of the level crossing is not mandatory if no train is approaching, the differing speeds of road users have to be considered: Whereas the sighting distance lA increases with increasing speed of the road user, for the ap-