01 A traffic light for changing sides
The detail is easy to miss because the crossing is short and anyone making it is usually thinking about paperwork. But it is there: traffic on the bridge runs on the left, as in Thailand, even though the bridge lands in a country that drives on the right.
The switch happens at the Lao end, before the border post, and is controlled by traffic lights. That is to say, it is not a manoeuvre each driver sorts out alone: there is a small, purpose-built piece of infrastructure dedicated to letting two different driving conventions meet without colliding.
02 One thousand one hundred and seventy metres over the lower Mekong
The bridge is 1,170 metres long and opened on 8 April 1994. It was the first ever built across the lower Mekong, and it links the Thai province of Nong Khai with Vientiane Prefecture. It stands downstream of the city centre, not on the river frontage: you cannot see it from the riverside walk.
Seen from the bank it is a sober piece of engineering: a continuous deck on concrete piers, with no towers or cables, crossing a river that in the dry season leaves half a bed of sand exposed beneath the spans. Nothing in its appearance announces the oddity it carries.
Two countries, one carriageway and a railway line down the middle. The bridge resolves all three at once.
03 And a train runs down the middle
A metre-gauge track from the Thai station at Nong Khai runs along the bridge's central reservation. There is no separate deck or level for the railway: the rails are set into the same pavement the cars use, separated from the lanes by little more than a painted line.
The consequence is the obvious one: when a train crosses, road traffic is stopped. The bridge cannot do both at once, so it alternates. It is an unusual arrangement, and the reason a border crossing of barely more than a kilometre can take longer than its length suggests.
04 What it means if you cross it
In practical terms, the important thing is that the bridge does not work like an ordinary road. There are two border posts, one at each end, and between them the crossing is made by vehicle rather than on foot. On top of that, traffic may be held if a train happens to coincide.
And there remains the detail that gives all this its title: at some point in the crossing, without anyone making much of an announcement, the entire flow of traffic moves from one side of the carriageway to the other. It is worth watching for. This is one of those administrative seams that normally stay hidden on a map and that here are resolved in plain sight, with two sets of traffic lights.
