I have some real rough patches of back yard that have no good grass, just weed grass and packed dirt. Years of shade, runoff, and traffic. Work area, welding area. I just started tossing out some grass seed, knowing I am wasting my time. But the seed should be used...

Usually grass seed doesn't take there. I don't do much to help it. Now I'm thinking overseed daikon instead, and then in the spring, try grass seed. Anyone ever try daikon as a step in reclaiming decent lawn?

Daikon will definitely break up hard soil but it's getting kind of late to plant that in PA since frost is only a couple of weeks away. Frost will kill the plants before they have time to do much. I planted daikon radishes all over my garden one August to break up the clay. They radishes got huge and did help with loosening up the soil but frost killed the plants.

I went ahead and tried this. The daikon hasn't done as much as I expected it would. Some seed did germinate, but not nearly as much as I expected has taken root. Those that did have not gotten much beyond the cotyledons, 2" high at most. The area has experienced the usual washout and is still being trod upon. Some grass I planted earlier is sprouted decently but as usual it is very patchy and uneven.

The prepped raised beds have daikon between 8" and 15" high, so I was expecting more. The stems are probably more fragile than I realized, and getting broken with walking. It's not as though I walk in those.

Been meaning to update...
I would say this did not work at all. The radishes never took off last fall on the bare patches in the lawn. They grew in the other spots I planted, but not there. Some germinated, but very few, and they never got big. In spring, some more started to sprout but again nothing big. Some radishes in other spots made it most of the way through the winter.
This fall I should be able the area re done, I hope, but I was kind of disappointed that a seed named "digger" did so little.

Seems like only a few things will grow well in high traffic areas. My favorite weeds for that kind of ground cover are plantain and hawkweed, although there are others that will stay small if the ground is hard and always walked on. Plantain starts to get tall as well if the ground is any good, so it might not work. Hawkweeds spread by runners and form a mat, it's as good as grass for walking on or working on. You can whippersnip the flower stalks if you want to keep it clear and low to the ground.
Not exactly lawn, but it's a green cover.

When the ground has been heavily compacted.... it's gonna be difficult to grow anything without getting in there and breaking up the compaction.
They used to call plantain... "white man's footsteps" because it was about all that grew in the wagon ruts...

Maybe you could grow some rumex and chicory?

Up in PA, with all the natural rainfall, you should actually do better than down here with the heat and droughts... which create hard-pans without any traffic...

If you aren't actually going to grow anything useful, I'm not sure I'd expend any effort on the spot.

We started a garden inside the foundation of a huge chicken house.
The soil was so hard we had to run a sub soiler through it several times.
Some might call it a ripper but it's just a long tine digging way down deep to break up the soil.
Ours went down about 2 feet.

Can you broadfork the patches? Have you considered removing 6 inches of soil and replacing with good compost/soil? Send in a soil test,too.
White clover may do well once the soil is right, or try doing a year/two years of cover crops such as winter rye grain mixed with Austrian peas or hairy vetch. The vetch is very cold hardy and I think it can tolerate some traffic.
Consider nixing the grass and making perennial flower beds?

No flowers, it is the highest traffic spot in the yard, sees many feet, man and dog, daily. I had broadforked it before seeding, both grass and daikon, but not real deeply. The grass takes in some spots, but then dies back.

I was very surprised how little the daikon seed took to that area. The seed grew as expected as cover crop in beds, so I guess there was not enough sun or warmth there in October to get it started.

I second the White Clover. It grows so readily here in PA. I don't treat my lawn for it for so many reasons.

Another method you could try, is put down a thin layer of cheap peat moss when you seed. Water it down real good. Then let the roots do the rest. The hardest part is just getting then little guys to take root.

They have those straw mulch blankets now at TSc and Lowe’s. Might help to get things established but I think you may be better off with a gravel or wood bark walkway.