DS is 14 months old. The interest in cars started about 2 months ago.
When will this obsession wane? Or is this likely to be a lifelong thing? 
I live in a busy part of London and when we're out, the running commentary from DS is "Car! Car.....car. CAR! Beep beep! CAR!" and also "Car! Whee." (i.e. wheels) It's very sweet, but I'm not sure how much longer I can keep up the pretence of being fascinated by cars....
Of course, DH is happy because DS loves watching Top Gear with him.