I guess if you want to see prehistoric cart ruts then Misrah Ghar il-Kbir aka Clapham Junction is pretty much the place to go. However cart ruts are not just a Maltese phenomena, they can also be found in Sicily, Spain Sardinia Greece and France but I guess the reason why the Clapham Junction cart ruts have become so famous is that there are so many of them in such a small area.
The site can be a little tricky to find especially if you are coming at it from the Dingli cliffs. Your safest bet is to get on the Rabat to Busket gardens road and look for an area between two large modern quarries.
The cart ruts themselves are pretty remarkable when you consider their age. I should mention something here about the age of the ruts. There is a lot of debate on this subject. There are a number of rock-cut Punic tombs in this area and at least one of the tombs cuts through a set of cart ruts. The tombs have been reliably dated a have been dated to the later centuries BC. This means that at least some of the ruts are older than this date. Most of the experts seem to agree that most of the ruts are pre-Punic but opinions differ from this point onwards. Some argue that the ruts are from the temple period whilst others, including Trump favour a Bronze Age date.
The debate around the use of the ruts isn't much clearer than the dating debate, however many experts seem to favour the idea that the ruts were made by some kind of vehicle but after this there seems to be all sorts of opinions. Trump states that 'Wheeled vehicles do seem to attract the least serious objections'.*
As a visitor I guess you could probably spend the best part of an hour following the ruts around this limestone plain, they are fascinating but I'm going to contradict myself here and tell you that I found them to be are pretty unremarkable and felt quite under whelmed. However, what I really did enjoy about Clapham Junction was the Ghar il-Kbir caves and the rock cut Punic graves.
*Malta Prehistory and Temples
David H. Trump
Malta's Ancient Temples and Ruts
R. Parker & M. Rubenstein
Institute for Cultural Research Monograph Series No. 26