You are encouraged to use public transport to leave CERN. The bus and tram are just outside of CERN. The Y bus stops at CERN (direction Ferney-Voltaire) and goes directly to the airport.
The 18 tram can be taken from CERN to Blandonnet, and then you go down the lift of the overpass to the road below where you take the 28 bus (direction Jardin Botanique), or the 57 bus (direction Aeroport) or the 23 (direction Aeroport). You can also walk from Blandonnet stop to the Terminal (15 minutes).
For the times that best suit you, please use the Trip planner ‘From: CERN To: Airport’.
The trip planner can also be used to find the times of the tram to ‘Cornavin' train station (the 18 tram goes directly there).
Please do not forget to purchase the tram/bus ticket ‘tout Geneve’ at the machine on the tram stop.
A taxi from CERN will cost about 35 CHF to the airport and about 45 CHF to the railway station in Cornavin.
There is a free CERN shuttle service which goes to the airport from the CERN hostel (building 39) and from the Main Building (Building 500, just below the Main Auditorium). However, this is a small shuttle (14 seats) and it is not possible to add an extra bus. Therefore public transport is a safer option.