MOST LAW-ABIDING TAXPAYERS do not spend many waking hours worrying about prison conditions. But today, the care and feeding of prisoners is far more than a bleeding-heart cause: Prisons are jammed, and unless prompt changes are made in determining who should be incarcerated and for how long, costs to the public will be enormous. In Maryland, Virginia and the District, efforts to expand facilities have reached their limits. Instead of the tra ditional build-more-prisons approach, corrections of ficials are turning increasingly to an emphasis on prudent ways to reduce their prison populations. These efforts make sense for many reasons other than cost, but a look at some projections underscores the point:

Virginia's director of corrections, Terrell Don Hutto, says the overcrowding of prisons is becoming so severe that unless changes are made, construction could cost more than "taxpayers can reasonably sustain." Virginia is among the top 10 states in average sentence length - about 28 months - and ranks high in incarceration rates. Currently there are about 9,600 inmates in a state system with an 8,100-bed capacity; some 300 are in hospital beds or other intransit facilities - and 1,200 are in county and local jails awaiting space. Most of those in state cells are doubled up in tight quarters similar to those that courts in Maryland and the District of Columbia have deemed are cruel and unusual punishment.

Virginia has been trying to keep up, too: Since 1975, the state has used everything from house trailers to warehouses and an old mental hospital to pull together about 2,600 beds. Nearly 700 more are being readied. But without other changes, estimates are that the state will be 8,000 beds short by 1990. The stories are similar in Maryland and the District. Corrections officials cannot control the number of people judges send to them, but they can relieve overcrowding through earlier paroles for those convicted of nonviolent crimes, halfway houses and community service programs. For example, Maryland is granting 10-month-early paroles to about 500 inmates by Aug.1; Virginia will have released about the same number by the end of this month. Virginia officials note that if this kind of program were coupled with a 4-month decrease in the average sentence, the prison population could be closer to 7,600 in 1985 - instead of a projected 13,000.

If these changes are to succeed, however, they will need a far greater understanding in the state legislatures, where many lawmakers still find it politically easier to portray supporters of any new approaches as advocates of indiscriminate and dangerous releasing of criminals. But the jam behind the bars continues, and something's got to give - for reasons not only of humanity and cost, but also because too many of those crammed in prisons would be better off somewhere else.