Disclaimer: if you do Something NOT-SO-SMART™ and become ill, injured, or die...that's your choice, not my fault or responsibility. Beware of snarkcasm.
While there's a lot of good rules of thumb listed, the tests to be concerned with are the same as with any meat, at any time: does it smell and look edible/fresh? Length of time is a consideration, but let's get some reality into the picture. Anything killed that day (OK, probably not aliens. And if you find fish on a road, avoid them just because eating fish found on a road constitutes Something NOT-SO-SMART™), even if it sits out all day is usually, generally speaking, edible. In warmer weather flies laying eggs are a concern, in cooler weather, not so much. Thoroughly cooking the meat solves the flies/eggs concern, though. Meat can hang for a few days...hunters and farmers in different cultures around the world have done this for millennia. Usually gutted but, sometimes, even the innards were left in. It even allows the meat to break down a bit and improve the taste.
There's even evidence that humans were doing things like killing mammoths, sinking the unused portion under water in a lake or pond to freeze, and revisiting the carcass the following Spring. If the innards are intact in roadkill, you want to remove them as soon as possible. It should go without saying that you want to wash the meat, keep it cool, and cook it as soon as possible. If intestines, bladder, or musk glands (on feral hogs, wild boar, or javelina for example) were crushed into the meat (we're talking really bad collision here and/or probably run over again), you should probably avoid it.
The ultimate modern reality check is how the world's arguably most effective special operations unit to ever exist trained its recruits going through selection. The Selous Scouts of Rhodesia had mandatory training wherein baboon carcasses were hung from poles the last 3 of 5 days that the recruits were denied rations, to show the extent to which a man could go to survive. They would then butcher and eat the rotting, maggot-infested meat by cooking or boiling it after had sat in the sun in front of them for three days. However, they would not reheat/reboil/recook such meat a second time, as rotted meat apparently becomes deadly if already cooked once and begins a second spoiling phase.
I myself have eaten game that was allowed to sit out a few days (seasoning), as many hunters here in the USA have done over the years. I have even eaten wild meat that was spoiling (did not have the maggot infestation/issue though) after cooking it, in a similar manner, and am no worse for the wear. But I also do stuff like eat fresh charred scorpion off a campfire rock, and I'm not saying your average foodie is going to enjoy the experience of dining on meat that's going/gone bad and been cooked. Just that within reasonable timespans of limited spoilage, they aren't necessarily going to die (but don't do Something NOT-SO-SMART™ just to say you ate days old roadkill, either). Doing Something NOT-SO-SMART™like that, is always a risk. [I'm a hunter and former infantryman who has taught emergency preparedness and survival, so risky stuff and eating things most folks wouldn't normally prepare on the stove is not probably as big a deal to me as it would be to the average person just wanting to get their waste not, want not dietary exploration on.]
If you know the roadkill appeared that day, you are fairly safe, but environment and other issues are always 'current situation' elements to take into account. What matters in that case is not so much how many hours it has sat out, but whether or not it's mashed into pate, contaminated, or if you can stomach it. An excerpt from an article worth reading...
"There is absolutely nothing wrong with rotten meat, even it it's crawling with maggots, providing you cook it first and eat it while it's hot," asserts Sgt. Maj. Anthony White
From Ludington Daily News - Apr 27, 1977
Ex-Marine Says Rhodesia's Brown Beret Best in the Bush