Temperature ranges from 20 °C in winter to 35 °C in summer (sea level). Rain season is in summer. And it comes rather suddenly, rains hard and stops quickly. Don't walk away from a shelter in summer.
The east side of the island is rainier than the west side (that is very dry).
Its very dangerous to hitchhike in Reunion cause the guys there are very frustrated and can rape you. There is a lot of criminals in the mountains also.
The road system is developed on a European standard. There are no motorways.
There are no trains yet. The backup solution is called "car jaune" (yellow coach). A quite inefficient coach system. They're all yellow, easy to spot.
The ring road
One road goes all the way around the island. It is an expressway from St Benoit (East) northward to St Denis (north) and then to St Paul (West), St Leu (West) and St Pierre (South-West). This road is always easily accessible from the various city centers using the appropriate bus network or sometimes just walking. You can hitch on the access roads or roundabouts.
The St Paul-St Leu section is still not express, but it's planned on opening in June 2009.
The island has the shape of a cone. From the ring road, you can catch smaller roads uphill that take you in the highlands. Those are overly easy to hitchhike, since they are rather slow.
Be careful, it rains easily in the highlands, on the east side in particular.