法律是這麽規定的。你收到的綠卡或者welcome notice

（具體哪一張我忘了）後麵寫清楚的，你必須一直攜帶，而且警察有權讓你出示。
但是實際操作中，大家都不把綠卡帶身邊的。也幾乎很少有警察想到會要看你的綠卡，除非一些邊關的警察等。你可以make a copy of your green card and take the copy with you. At least it has A# on it so if necessary, the police can check the A# in his computer --- that's better than having nothing to show them.