Much easier dissipating heat when you're moving. Air's coming at you, fan is spinning faster, etc.

Were you running the AC?

Have you figured out where the coolant came from?

Is your water pump making any noise?

Unless you've lost a bearing and the impeller isn't spinning, it's unlikely to be your pump, which means it's doing its job moving coolant. The next link in the chain is the radiator, whose job it is to cool said coolant. Then there's the thermostat which, etc etc etc...

You've got a 240 so no fan clutch. It sounds to me like a marginal radiator, though I'd want to be absolutely sure I'd isolated and eliminated the coolant leak before you go chasing more exotic points of failure.
There's always the possibility of a faulty thermostat (I had one fail after a year), though you'd likely be seeing signs of that at speed.

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1981 240D 4sp manual. Ivory White.