Taxis, while seen occasionally, are not seen in abundance. Do not count on hailing a cab on the street, but if you see one, you may still try to hail it. In order to get a cab you will need to call ahead and wait for one to pick you up.
The brands seen the most are ICare Sedans & Cab co. and Independent Taxi in downtown although BATS Taxi has been mentioned here on TripAdvisor in the user forums, even though they have bad reviews and are mostly on the beach, as that is where their headquarter is located.
Your hotel can recommend the best to call for that area and will probably have the number.

If you plan to take in a lot of attractions, a rental car is the way to go. The average rental is not exorbitant and parking is usually easy to find and relatively inexpensive.

The city of St. Petersburg uses a grid system of roads where the roads going east -west are called avenues and roads going north-south are called streets. Watch out for whatever comes after the street or avenue name though because it indicates where in the city that road is. For example, there are two 22nd Streets - one 22nd Street North and one 22nd Street South. First Street runs right through the middle of downtown. 22nd Ave N becomes 22 Ave NE on the East-side of 1st St N.

There is one main interstate that runs through St. Petersburg and that is I-275. It comes in from Tampa to the north and continues across the Bob Graham Sunshine Skyway Bridge, currently the world longest cable stayed concrete bridge, to I-75. People with high bridge phobias, be warned. This bridge takes about ten minutes to cross and is 193 feet over the waters of Tampa Bay. Branching off from I-275 going east is I-175 which will take you to Tropicana Field and downtown and I-375 which will take you to the Pier and downtown.

If you are coming to St. Petersburg for the beaches off the barrier island, all you need to know is US19. Find that and you are set. It goes the entire length of the barrier island which is sometimes so thin you can see water on both sides. The route to US19 is well marked. Use those signs to get there because US19 is not the same as 19th street or Alternate-19.

There is not a huge rush hour in the morning or evening - traffic usually continues at a brisk pace of around 60 or more miles an hour. The speed limit is taken as more of a suggestion as seems to be customary in this part of Florida. The biggest problem with driving here is that some people will go even less than the speed limit while just as many (usually more) commonly drive between 60 and 80 (on the interstates). 85 or 90 is also not unusual, though quite illegal and certainly unsafe.

There are not many toll roads in St. Petersburg but there are some worth noting: the (only) road going into Fort DeSoto is a toll on the way in and the Bob Graham Sunshine Skyway Bridge is also a toll before crossing in either direction. The tolls are low though - currently fifty cents and one dollar respectively. The main toll road, the Selmon Crosstown Expressway, is in Tampa so unless you have extensive business coming and going to downtown Tampa during your stay tolls won't be an issue in St. Petersburg.