Cannes is at a crow flies from St Tropez, thus, this road is always crowded during the high season.
You have to be careful at what time you plan to get in one city or another in order no to get stuck in the traffic.
The Cannes film festival is also a good occasion to visit St Tropez because the spring season in one of the best moment to walk in the St Tropez village.
Cannes / St Tropez: 86km (53,5mi)
Basic timing for this journey is about 1hour 15min, hour by good traffic.