The Transport Authority says a high threshold will have to be met for any highway or motorway to have its speed limit lifted to 110 kilometres an hour.

The NZTA is looking at varying open road speed limits by up to 50 kilometres an hour under a new classification system.

The changes could be in force by late next year.

Road safety director,Ernst Zollner, says long straight highways like in between Ashburton and Christchurch may not meet the requirements.

He says in order to lift speed limits they have to be able to avoid the risk of head on collisions.