so how come he's legally driving at 159?
Of course it's not legal to drive 159 km/h. The maximum speed allowed on UAE highways is 120 km/h. The speed cameras in Dubai take a snap if you exceed that by 20 km/h, ie at 140 km/h. However the speed cameras in Abu Dhabi are a bit more "tolerant". They will take a pic at 160 km/h.
I guess that's why Aisha sees 159 as "legal speed".