I thought it would be to easy tooncheese is correct the pitlane speed limit was first introduced after the 1994 San Marino GP after an incident where a rear wheel detached from Alboreto's Minardi and injured several Ferrari mechanics (and one from Lotus).
This was the race where Ratzenberger died in qualifying, Senna was killed in the race itself, Barrichello was lucky to survive a high-speed qualifying accident and a Lamy/Lehto accident sent a wheel into the crowd at the start.
A bloody disastrous race weekend all round really.