If you don't see the difference, you may smell it.
In three months, there will be no single-decker buses on Hong Kong's roads that pre-date the European Union's emission standards.
Their retirement precedes that of their double-decker counterparts, which will be gone by the first quarter of next year.
The last pre-Euro single-decker still operating is Kowloon Motor Bus' Dennis Lance model.
'It served both the old Kai Tak airport and Chek Lap Kok airport,' said veteran KMB driver Chan Wai-hung, who has been driving the model since it was introduced in 1993. 'I am happy it will soon rest in peace.'
All 400 existing pre-Euro buses have been fitted with filters to ensure that suspended particles in their emissions meet Euro I, the original and lowest standard. But they emit 18 times more of pollutants such as nitrogen dioxide than Euro V buses.
KMB principal engineer Kane Shum Yuet-hung expects its Euro I buses to be phased out by 2014.
The company said 300 new Euro IV and V buses would be brought in this year to replace older ones.
Single-decker buses comprise about 4 per cent of KMB's 3,900-strong fleet and are seen mainly on little-used routes or where a height limit exists.