Help!! My 4 month old cats keep sneaking outside!

I have 2 sibling 4 month old cats. They have this habit of waiting by the door and running out when I open it. I have put collars on them with bells since this has been an issue since I got them. And in the beginning they would run out and stop by the flower gardens.
Now they get out and take off running. The other night they got out at 10 pm and luckily I was able to catch them before they ran too far. But I need to prevent them from getting out because I fear them running too fast and too far to be caught. Or worse, running into the road.
We don't have a fenced in yard and don't want to invest in that because we are looking at moving.
I am not sure what else to do. Any suggestions?? This has me completely worried that they will run off and get lost.

Any way you can confine them in a room when you're going out?

We could do that, however the litter boxes are in the basement and they need to go down the stairs which is by the back door in order to get to the basement. I would hate to put the litter box up in our small house, but I guess that would be better than losing them forever. I'm trying to think of alternatives to what we can do rather than putting the litter box upstairs.
I'm hoping that they will outgrow this eventually. Do they??
Thanks for your suggestion!

They should as they get older .........

For now I would make sure you are always coming in with something ......... like maybe a cut out carboard box or something blocking them off at the path as you enter .