IF PRESIDENT REAGAN has his way, the nation's highways are going to be busy. Two months ago he suggested that people who weren't happy with the high unemployment rates in their home towns should "vote with their feet" and set out for areas where the economic climate is sunnier. More recently, he had similar advice for the poor. If the poor find that the states they live in are insensitive to their needs, he remarked at his last press conference, then they too should exercise their freedom to move to more generous jurisdictions.

The president, of course, hopes that this won't be necessary. His "new federalism" policy would allow states to cut not only basic welfare for families but food stamps as well. Based on his own experience as governor of California, however, he is confident that states will not use this new latitude to treat the poor unfairly. This is a happy thought and, no doubt, one that will be justified by many states. State governments changed a lot over the past decade, and many can point to policies that compare favorably with any sponsored by the federal government. But those great variations in state treatment of the poor that Mr. Reagan apparently remembers as things of the past are still very much with us, and without federal guidance they are likely to increase.

There are, for example, enormous variations in the benefits that states now see fit to give to welfare families with children. In Mississippi, a family of four is supposed to live on a cash allowance of $120 a month--that's a month, not a week. In California, it could get five times that amount. Mississippi, of course, is a poor state, which explains at least part of its low payment. But other states, such as Texas, have no such justification. Texas is one of the few states with a substantial surplus. It provides a family of four with $140 a month.

Similar variations can be found in the whole range of programs for the poor. One state, Arizona, is only now thinking of running a Medicaid program at all. In recent years, federally financed food stamps have narrowed these differences somewhat, but, under the administration's "swap" plan, states could divert food stamp money to more popular purposes.

The president is right, of course, that the poor can move to states where they are treated more generously. They have, in fact, been doing it for years. The result hasn't been very attractive for the receiving states. The poor came for jobs, but when these weren't available they settled in the congested urban ghettos, where crime and hopeless poverty are bred. The generous states were repaid for their kindness by crushing burdens on local tax bases. Now, to add insult to injury, many of these states see their jobs slipping away to the very areas that once sent them their poverty problems.