When driving on a regular street, is there a minimum driving speed that you can’t drive under by law?
PA doesn’t assign a minimum speed except on limited access highways, but §3364 (a) relates to impeding movement of traffic by operating at an unnecessarily slow speed.
And who decides what is considered to be “unnecessarily slow speed”?
I would gather that would be determined by the officer. I think assigning a numerical value to it would lessen the impact of the law. If it is sunny and you’re on a wide open stretch of road, going 40 MPH might be considered an “unnecessarily slow speed”. But if it is snowing and close to zero visibility, going 40 MPH would get you a reckless driving charge before a slow driving ticket.
In part I think it’s to keep mopeds/scooters off major roads. And, driving too slowly can be as much of a danger as speeding.
Not only that, but it would give an officer to pull any car he wanted over and site that the driver was moving at an “unnecessarily slow speed” as the reason. All of a sudden, the police can do all of the racial profiling they want to.
Huh? 
Many, many people speed. Who’s driving too slowly? The elderly? Police might try to falsify things I suppose, but why would they choose “driving too slowly?” How did we leap to racial profiling? Is there some ethnicity or nationality that usually drives that way?
No, what I’m saying is that any cop can pull over a car going 40 mph driven by a black person.
Or a teenager. Or a woman. Or an Hispanic. Or just about anyone.
He has to justify the stop. As noted above, driving conditions determine what is safe and what is not. if a black person were driving 40 mph in a 40 mph zone in clear and safe conditions, no cop is going to cite him for unsafe driving based on his speed. It would never hold up in court. It is not probable cause for a stop.
The reason that I asked the question is because I ride my bike all over the place and I know that bikes must bike on th street and obey all laws that a car does. Where I live, there’s a “3 foot rule” that states that when a car passes you, they must give you at least 3 feet and sometimes when I’m going up a steep and narrow road, I can’t bike very quickly (sometime 2 or 3 miles an hour) and cars are not to pass me. I’m just not sure what to do in that situation.
Here’s an excellent discussion of that law as it applies to bicycles in Minnesota (and most states) and the short answer is: don’t worry about. The law intends for you to ride your bike on the road and they know you can’t pedal at 40 MPH. You’re still well within your rights to be on the road enjoying a healthy, fun, economical, and safe means of transport.
So does this mean I should pay absolutely no regard to the legion of angry drivers behind me that are honking at me and verbally making threat against my life because traffic is backed up for half a mile because I want to enjoy a healthy, fun, economical, and safe means of transport?
If you lived in Portland, Oregon, yes. 
Every vehicle on the road has to obey the rules of the road. If you are impeding traffic, you can be ticketed, regardless of the mode of transportation.
That doesn’t make an sense. Why should I ignore all of the drivers behind me who are honking if I’m the one who is impeding traffic?
If you’re on a bicycle, shouldn’t you be on the SIDE of the road, so as NOT to impede traffic whilst going up that steep hill?
Again - someone is looking for law to solve a problem that common sense can handle. If you’re aware that you’re impeding traffic, angering drivers, and being a problem to the flow, get the ##!!@ out of the way. Just because something isn’t illegal doesn’t mean it shouldn’t be avoided. Presumably, it wouldn’t happen to you all the time, every day, the entire time you ride up that hill, but pulling over so that traffic can clear is a wise and courteous thing to do. What difference does it make that there may be some law governing the situation? If you’re in your car and you see that you’re going slower than the traffic behind you, don’t you pull to the right so that they can pass? C’mon.
Is the side of the road paved? Or is it sandy, rocky, uneven, etc.? Does it lead to an embankment?
A cyclist should not have to sacrifice her safety so that motorists can get to their destination two minutes faster.
Sorry, I meant to say " the right edge of the pavement that is not sandy, rocky, or uneven ,etc." instead of the confusing “SIDE of the road”.
Bicycles are vehicles, so they shouldn’t be on the shoulder. Though I think it’s tolerated in most places, and even allowed in some.
The law in most states say the bicyclist must ride as far to the right as is practicable and safe. That generally means a cyclist should be in the right-most lane (unless preparing for a left turn). If the lane is wide enough that it can be safely shared with a car, the cyclist should be on the right edge of the lane. If not, the cyclist should use the whole lane.
I don’t know about elsewhere, but here in the U.K., drivers of slow vehicles - which includes bikes - are supposed to pull over now and then to let traffic pass.