This is what we call a 'radar'. Each bus daily has a sheet of paper like this one for the driver to check. It shows the bus work (here bus number 04) from it's start at the depot in the morning, to it's way back at night, and the several duties and drivers (here 14, 2, 9 and 23) who will successively drive during the day. It also allows the driver to regulate his speed to pass the several regulation-spots at the right time (if possible).
The regulation points (theorical) mentionned are: PO for Porte d'Orléans terminal, OB for Observatoire Port-Royal, CH for Chatelet and GN for Gare du Nord...
Are also indicated the eventual special operations the drivers must do (park, change bus, etc...)
According to the period of the year, and also the day of the week, the 'radar' isn't the same. This one here was valid during the school holidays (vacances scolaires TM50) in february, 2001, from monday to friday (LAV):
Today, this tool is supported with a new system of regulation by satellite called "SIEL", with a screen for the driver to check his delay or advance for regulation, as well as the time between previous and next buses.