EVERY ONCE IN a while we try to blow the dust off that backroom shelf in city hall where good and bad government ideas get stashed for good and bad reasons. Today's effort brings to light a proposal that Mayor Barry says he'd like to see move but that always ends up under study or under wraps: the plan to build a new prison in the District of Columbia. This is the project that the federal goverment offered to help with money and land; and this is the project that Mr. Barry finally endorsed and then -- true to bureaucratic form -- turned over to a study commission. Instead of recommending a site, most commission members have preferred to hope that the idea would go away, that nobody anywhere would have to worry about a prison next door or down the street.

Instead of talking sites, the commission has been talking alternatives to incarceration -- a fine subject, one the administration should deal with as it also deals with the pressing issue of where to build some kind of new facility. But so far the commission has not concentrated on specific sites -- even though it has before it a list of federal sites in every city ward.

Perhaps there will be some focus on this issue when the commission holds public hearings on Monday. That should be the subject addressed by witnesses: where to build, and what kind of facility? There is legitimate concern that Ward 7 or 8 will wind up with the new facility; that is a pertinent matter. As we have said, no ward in the city should be arbitrarily or politically excluded from consideration.

But at last report the commission wasn't expected to make any final recommendations until January. Why so long? If it takes time to inspect each site and review its potential, the public should be advised and kept abreast. Mayor Barry shouldn't be left waiting; on the contrary, he should make sure that this proposal never gets restashed on that city hall shelf. The new prison is a matter of public safety that should be addressed expeditiously.