A main signal and a distant signal of the
<a href="http://www-users.rwth-aachen.de/christoph.schmitz2/smsignal.html">semaphore
system</a> showing Hp1 (proceed) and Vr0 (expect stop) in Aachen Rothe-Erde on
13 July 2001.
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Digital photo by
<a href="http://www-users.rwth-aachen.de/christoph.schmitz2/index.html">Christoph Schmitz</a>