Lights that indicate free parking stalls
Sometimes when I travel I see a great idea that hasn't yet spread everywhere yet. A parking garage I parked at in Tel Aviv had LEDs visible on the roof above every stall. These were red and green if the stall was full or empty. So it was quick to find an empty stall. This probably makes the garage more efficient because people don't have to circle hunting for a spot, and this justifies the cost. (The main cost of these is probably wiring the power for them.)
I've seen studies claiming that in busy areas, up to 30% of the traffic is cars circling looking for parking. Mostly they are looking for free parking or convenient on-street parking, since parking garages, though expensive can usually be found and entered quickly. Indeed, while on-street parking is often much more convenient, in many cases this is an artifact of parking being subsidized (because it's free, or free to people who live in an area) or cheaper than commercial parking markets. But we don't seem ready to fix that, though many cities put restrictions on street and metered parking, limiting the number of hours so that it is in theory only for visitors rather than all-day parkers.
There are many companies trying to see if they can improve parking using mobile devices and the internet. There are companies with sensors that manage parking spaces, companies that let you find spaces on a mobile device and even enter a garage with your mobile device. In some cases you can even extend your parking (if you prepaid) over the phone. Cities have been moving away from traditional meters to things like block meters (where you get a ticket and then put it on your dash) or fancy enforcement vehicles with licence plate cameras that spot not only if you are in a spot too long, but if you move within the busy zone to another spot.
As a user of parking, I would like to know I've got a good spot lined up before I get to my destination, and just pull right into it. I want a competitive market but I don't want to waste time and gas hunting. There are companies trying to address this, though mostly in commercial lots. It's mostly pretty basic right now -- it's considered fancy to even have sites like parkopedia or bestparking with a database of the parking in a city with the prices so you can comparison shop the parking lots.
So now for some rambling on what might be done on street.
On-street parking is particularly hard, particularly in free-parking zones where the spots are not defined. You don't want to define them, since you get more cars packed on the street if there are not allocated 20' spaces. The parking space LEDs are hard to place on the street -- they could make sense on top of meters, but cities are moving away from meters when they install new parking infrastructure.
I have been wondering if you could allocate street parking on smartphones without much need for enforcement. That is to say, the users would be the enforcement.
For example, the parking computer might manage numbered spots on a street. Driving in, you might request and prepay for a spot. However, you would have only a certain amount of time to occupy it. 15 minutes after your ETA if you haven't filled it, it would be freed up again with no refund. (This discourages people just buying up all the subsidized street spots in advance or reselling.) You would go to the spot and write a number on a slip of paper and put it on your dash or window. Once your time was up (you might be able to extend it if you do it early enough) your spot would be sold to somebody else. When they arrive, if your car is still there, they can note your code number. If it matches, you are fined, and you pay for them to get free parking somewhere nearby. (See below for discussion of gaming this.)
Once a stretch is sold out, the parking kiosk would illuminate a red light up high were all can see it. If you see an open spot in a sold out block, you can take it, then quickly check the status of the spot at the kiosk. If it's an overdue parker, the spot is yours. If it's a parker due to arrive shortly, you either have to leave, or you can check if any other free spots are overdue parkers. If so the arriving parker can be informed by mobile device they should take a different spot, or you can move.
When you arrive, if there is somebody in your spot without the valid payment code number, you can enter their plate, and a parking enforcement scooter can come to ticket them, and possibly tow them. You have to park elsewhere, but if they are still there when the enforcement scooter comes, it can be on the illegally parked car's dime. If they leave before then, you're out of luck, otherwise people would just make fake claims. But this is the only thing the enforcers have to do -- respond to actual complaints. If nobody wants a space, and you overstay, you aren't charged or ticketed -- but that makes sense to me. Why ticket when spaces are not contested?
This could even be done without fixed sized spaces. A number could be marked every meter on the curb. The length of your car would be stored in your account, so when you park you could enter a fractional meter for your front bumper and the system could figure things out. While your account might well be tied to your car length, otherwise it could be anonymous, as long as it has a balance enough to pay any fees and fines, though unless data is erased after all is clear, your parking locations will probably say a lot about you.
One way to game this would be to notice people who are parked and right down their code numbers. You could then later report those cars as still present after you purchased a spot, since as I said that gets you a free spot at their expense. This requires you pay for the spot, however, and it's a complex thing to do with minimal benefit, and the other party is going to file a complaint on you. You could only do it a lot if you had disposable anonymous accounts -- and those need not exist because anonymous accounts would have deposits on them.