New Zealand's One Lane Bridges

In 2009 New Zealand had 2,174 rail bridges and over 15,600 road bridges. There were 150 rail tunnels, totalling more than 87 kilometres in length. From 2000 to 2008 an average of over $9.7 million was spent annually on new road bridges.

Mountains and rivers

The high number of bridges and tunnels is due to New Zealand’s difficult terrain. Travelling inland involves crossing mountain ranges and gorges. The high rainfall and steep hill country create many rivers, which become wide and braided on the plains. The difficulty in crossing these rivers led to drowning being known as the ‘New Zealand death’ in the 19th century.

 http://www.teara.govt.nz/en/bridges-and-tunnels/1