So there's a highway (or tollway I guess) near me that's still considered new but not brand spanking new (about a year or so old).  The funny thing about that road is that there are no speed limit signs anywhere on the road.  There are speed limit signs (55 MPH) on the frontage road, but not the highway.  I didn't think much of this until I saw a cop looking for speeders (first pass, he was just waiting to catch someone, way back, he had someone pulled over).

How am I expected to know what the speed limit of that road is?  Here in Texas we have another tollway that is 55-60MPH, and highways that range from 60-70MPH.  Obviously there are enforcing a speed limit, I just have no idea what it is.

The limit is anywhere from 0 to highest speed you can go.
Play safe and choose a low number :)

That's what I use to do but now I see cops on that road all the time.  I generally go about 60-65 and just let people go faster than me but it just bothers me there are cops on that road all the time now and no posted speed limit.

Well from what my mom taught me (Go Ahead and Laugh) GENERALLY speaking Highways are typically 65 MPH. PARKWAYS (one of the dumbest inventions in our history IMO) were made so that they would be 55 instead.
Neighborhoods LN, RD, DR, CT, Cir are generally 25-35. ST and Ave are typically 30-40 and BLVD are 35-45.

As a new recruit, the rules are 50km an hour in the city, 80 km outside of it. Might vary with where you are though.

Are there no national or state speed limits in USA?

In the UK the national speed limit is 70MPH but it really is pot luck whether you get a ticket for going over any speed limit.

Nope, it's up to the county or the city (I forget which) and even that isn't consistent.  That's why you get some highways that are 55 and some that are 70.

In BC They're 50kmh if its unposted in the cities. Not sure about the highways. I usually go around 150 te he

I think it's the State Dept. of Transportation that ultimatly decides.