Got the following from another forum :
Your automatic transmission drivetrain was engineered to be left in drive while the engine is running and the car is at a state of rest at lights, and elsewhere.
However, your transmission was not designed to be shifted between drive and neutral an extra however many times (thousands? tens of thousands?) over it's lifetime by people who don't leave the car in drive.
Your engine is fully capable of stress. At a red light, in drive, it is idling probably at 700 or so RPM's. It is relatively unstressed. There is very little load on it.
However, every time you shift a transmission into drive, you are jolting the drivetran (transmission, axles, mounts, etc). As well as actuating the mechanisms that shift it into drive in the transmission. Constant shifting back and forth will cause undue stress and wear on all sorts of components not designed for those repeated stresses.
So leave it in drive. Your engine is well up to the task of handing it!
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