Shall I use "have" or "had" in this sentence:
These lights flickered continuously like traffic lights which had/have gone mad.
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'Had' is the past tense of 'have'. Both are used within verb phrases to create the 'Perfect', or complete, tense. 'Have gone mad' means, at this moment, the action of going mad has been completed. 'Had gone mad' means the action of going mad was completed at some time in the past. I think either tense is valid in the expression 'like traffic lights ...' Native speakers usually use the most simple tense with more than one valid option, so I expect more would choose the present tense, 'have', than the past tense, 'had'.