If the HARM is shot at, say, 20 miles away, the RADAR truck would have just around 37 seconds to move out of the blast cone.
Errr... kinda. The problem is far more with getting an accurate fix on the emitter location than it is with programming that location into a weapon - after all, at 20 miles you're getting into JDAM range. They're cheaper than a HARM and give you a far bigger bang - so your emitter location is less critical.

As an aside, the advantage with ALARM is the extended loiter time during which radar sets will be keeping their heads down, rather than any fancy electronics. That's quite a few minutes, enabling the bombers to transit the area and be long gone.
"I wish to heaven the Iron Curtain was windproof" - Unknown British Soldier, Korea