One more thing: save power by turning the lights on only when a car enters the lot -- a timer shuts them off after a reasonable period of time.  Or, just light a green arrow pointing towards each row that has available spaces.  No need for the red lights, those rows are of no interest.

I like the idea of using timer to shut off light if no activity detected.

Also, completely agree with you regarding "no need for red light". Point well taken...

The requirement for sensors in every spot seems to be a weak point of the system (especially in areas with bad winter weather, they are going to be a maintenance headache) Have you considered using some thing like a RasPi and a camera and doing some video processing to identify empty spaces?

Thanks for the feedback.

I did thought of this and do agree that installing sensors for each parking spot requires maintenance. But, I think like metered parking that are installed at each parking spot, inductive/ultrasonic/optical sensors could be installed and maintained.

Alternatively, camera & video processing could be used but I think it will increase the cost considering HD camera & developing video processing logic, Also, during bad weather when visibility is poor this approach can be an issue specifically, if outdoor.

I wonder if camera installed at traffic lights can be used for this. Also, in most of the parking lots camera are installed for monitoring so same infrastructure can be leveraged for APSIS idea. Also, video processing needs to be developed only once and so this approach will speedup installation and avoid maintenance.

So, definitely this approach has few advantages and something to consider.

A servo and colored flags may use way less current. Servos would use current only on changes, while leds need to stay on to signal, and they are too dim on sun days.

A standard servo would use 1W to change a flag, while a 5mm led uses 20mW continuously.

Solar panels can't also produce a stronger light than what is around, and the system has to work also at night, so the system can't output much light unless has a panel  5-10 times bigger than the led panel.

I like your suggestion of using servo and flags and definitely would consider this option since this way you do not require continuous electrical power to light LEDs.

Regarding your comments on solar panel, please note that solar panel is not directly driving LEDs. Li-ion battery will be charged using power from the solar panel during the day time. Battery power will then power circuit board & LEDs. Alternatively, 110V AC power in combination of solar power can also be used for rainy and other days when sun light is not adequate to charge battery. Circuit can be developed to check the power from solar and if less than threshold then system will switch to AC source to power LEDs.

An option for this idea would be to have lights in the ground in the drive way between the parking lots and show a red / green light/LED to show if the parking spot is taken. Sure you can tell for yourself for most of the spots close to you and when there are several spots open together, but for those hidden open spots behind a large truck this might be handy. Also you can just glance at the road and your not swinging you head from side to side looking for that close open spot (or trying to spot that open spot in the next lane) and not particularly paying attention to anybody running/walking in your drive path. If your looking at the road at those lights you should be able to spot any objects/people that might get in your drive path easier.

Thought of something else, use the color yellow when all the front half of the spots are taken and the only open spots left are beyond the half way point from the front of the building the spots are for.

Thanks for the comment & suggestion.

My solution "APSIS" will indicate available parking spot using LED light for any free spot including hidden behind large truck or far in the lane and not visible to driver. Since the post with LED lights is on the entrance of the lane, you do not have to swing your head and look inside far in parking lane as well and so it is safe for pedestrian crossing the road.