concrete wall, each side containing two traffic lanes. The typical traffic flow has two northbound lanes in the east tube and two southbound lanes in the west tube. On January 28, 1981, a counterflow system was introduced to meet increasing traffic demand in the tunnel. A bus lane was also added to the approaches a few months earlier as part of the $2.5 million program. At peak rush traffic periods, a reversible lane system is used, with a series of swing gates deployed that direct traffic in one direction to a single lane, while increasing the other direction to three lanes. Morning rush has three lanes northbound (inbound to Vancouver) and evening rush has three southbound lanes (outbound from Vancouver). Construction and maintenance The tunnel is long and made up of six precast concrete sections (length: ; height: ; width: ). The sections were floated into position by barge and then sunk into a shallow trench that had been dug into the loose sand and silt of the river bed. The trench and tunnel sections were then covered over with a protective layer of rock— stones filled out on each side, plus a bed of stones on top. A structure located at each end of the tunnel houses the main ventilation and pumping equipment. Concrete retaining walls make up the approaches, which extend out about from the ventilation buildings. At its lowest point the roadway is about below sea level, making it the lowest section of roadway in Canada. The Fraser River flows into the Strait of Georgia about downstream from the tunnel. Due to the tunnel being designed and constructed in the 1950s, very little consideration was given to seismic factors. The river bed is a thick layer of sediment on top of bedrock. This sedimentary layer may liquefy during a major earthquake, leaving the tunnel with nothing to rest on, and thus vulnerable to total collapse. In recent years, as the awareness of the effect of serious seismic activity developed, an engineering assessment and subsequent retrofit project was initiated to increase the survivability of the tunnel in the event of a significant earthquake. This retrofit project started in late 2004 and had been completed in November 2006. In the interest of providing active protection for commuters from the danger earthquakes pose to the tunnel, an earthquake early warning system called Shakealarm was installed and commissioned in 2009. This