Complex sentence: This was caused by tunnel runners who had developed a predilection for the underpasses under the Barbican Estate, which are wide and spacious, allowing multiple cars in the tunnels at once.
Split into simpler sentences: This was caused by Tunnel Runners who had developed a predilection for the underpasses under the Barbican Estate. The reason for this was due to the fact they are wide and spacious meaning multiple cars can be in the tunnels at once.