[thedoc (OP)]

Jeanne johnson asked the Naked Scientists:
   If the new street lamps are so energy efficient why would the cities leave them on all day and night.. why waste energy?
What do you think?

[ingolange]

I think it's a waste, honestly.

[evan_au]

When I was living in Europe, in a compact country having a high level of nuclear power, the suggestion was that the power generation capacity was available anyway, so you may as well have bright lighting along the full length of all your highways, and leave the lights on all night...

The reality is that it takes more "smarts" to turn lights on only when needed.
- Streetlights in many areas are controlled by a photodetector. This simple and cheap circuit has been used for many decades, and allows the lights to turn on at night, and off in the daytime without a complicated season-variable clock. It will even turn on the lights in the daytime if there is a dark storm approaching. The problem is that birds poop on the photodetector (or the photodetector malfunctions), so they think it is dark all the time, and the lights stay on 24/7.
- The modern trend towards "smart buildings" have office lights that automatically switch themselves off at night. I have found it a bit disconcerting to be working late in winter on an unfamiliar floor, and the whole floor went black at 7pm. I had no idea where the switch was located to tell the computer that I was still there.
- Even more modern buildings use motion sensors to turn on the lights when you walk into the room, and turn them off when the room is idle. Unfortunately, this sometimes leads to more complex controls like "We are still here, but I want the lights dim so we can see a presentation more clearly..."
- Some Nordic towns are experimenting with sensors on streetlights, so the lights turn on as cars or pedestrians approach, and fade out when they leave. This works well with modern LED lights, but would not have worked with older gas-discharge lamps, which take a minute to warm up to full brightness.

Overall, the increased "smarts" use a bit more energy, but probably less than the lights consume from being ON all the time. They are one more thing that could fail, but if they reduce the amount of time that the light is "ON", they may actually extend the lifetime of the lighting module.

[alancalverd]

Some time ago I calculated that it would be cheaper for Cambridge University to leave all the lights on all the time, including incandescent bulbs, for the next 20 years, than to retrofit sensors, never mind LEDs.

Highway lighting is pointless. All vehicles are required to carry their own lights and additional lighting reduces their conspicuity at night, especially in a scattering mist or fog.

City streets do not require lighting in addition to shop windows, and insurers generally prefer shop windows to be lit all night. 

The case for lighting suburban streets is dubious: some studies show that more crime is committed when the criminals have a better view of their victims or goods.

But energy and lighting policies have never been based on rationality. 

[Atomic-S]

Although vehicles are required to carry their own lights, pedestrians generally aren't, and if there will be significant pedestrians, that argues for street lighting.  Also, pedestrians may not be as well seen by drivers using vehicular lights alone, so it is safer to have street lamps also. Shops may or may not remain lighted all night, and even if they are, their lamps may or may not impart satisfactory illumination to the roadway, and might actually impair visibility there due to distracting glare. This, of course, will be less of a problem out in the country where there are few lighted buildings as well as fewer pedestrians, so that the argument for street lighting out there would be less compelling, but it may also depend on the volume of motor traffic.  The cost of retrofitting street lights with sensors/LEDs needs to take into account the current cost of annual maintenance on existing lamps if they have to be replaced often, which involves not just electricity costs, but also labor costs. If an existing incandescent fixture can be replaced by an LED fixture, future maintenance on that fixture will be greatly reduced, making it economic to do so.

[zx16]

Some time ago I calculated that it would be cheaper for Cambridge University to leave all the lights on all the time, including incandescent bulbs, for the next 20 years, than to retrofit sensors, never mind LEDs.

Highway lighting is pointless. All vehicles are required to carry their own lights and additional lighting reduces their conspicuity at night, especially in a scattering mist or fog.

City streets do not require lighting in addition to shop windows, and insurers generally prefer shop windows to be lit all night. 

The case for lighting suburban streets is dubious: some studies show that more crime is committed when the criminals have a better view of their victims or goods.

But energy and lighting policies have never been based on rationality.

I think you're right - the present-day massive lighting-up of cities and towns doesn't have much rational justification. Perhaps it stems from an emotional human yearning to dispel the dark.  Darkness in past ages was disliked.  But there wasn't much we do about it, because we didn't have the technology.

For example, Ancient Rome developed quite a sophisticated civilisation, but their lighting technology was pathetic. Their "lamps" were just wicks dipped in a kind of gravy-boat of oil.  This was no real advance from the Stone-Age, where similar "lamps" occur.  Such lamps could not give any widespread illumination, and consequently Rome, and all other cities, were "blacked-out" at night.

This benighted situation continued until the 19th century, when first gas, then electric, lighting were developed.

With this new technology, we could dispel the dark properly, and we went in for it exuberantly.  Now, a city after nightfall could be, not just a dim miserable huddle of barely-perceived buildings - but a panorama of brilliantly-lit celebrations of human achievement!

This has a deep and thrilling appeal.  Anyone who's flown into New York at night on a commercial airliner, and looked out of the window at the vast and wonderful sight, must feel a sense of pride in the majesty of human scientific achievement.

That's why I don't sympathise with people who want to turn off lights to "save energy."

You may save energy, but aren't you betraying the human soul?

[alancalverd]

Not sure I share your final sentiment there. Lead-in and runway lights give me a thrill because they symbolise a small achievement for a mammal: being able to fly and navigate at night almost as well as pigeons and butterflies. Only bats can do better.

"Majesty of human scientific achievement?" Bats refuel themselves as they fly: can you show me an aeroplane that eats insects?

We have a long way to go, and city lights just make me ask "why?"   

[zx16]

Bats refuel themselves as they fly: can you show me an aeroplane that eats insects?

Aeroplanes with jet engines must ingest and in that sense "eat" lots of airborne insects during flight!
However I accept that's not a valid comparison with what bats do, because bats purposefully seek out insects as "food".

Whereas aircraft don't fly around looking for insects, as they don't provide any "food" for the engines.

But on consideration, could engines benefit from insect-ingestion?  I'm thinking of the calorific value of insect bodies, when they're drawn into the combustion-chamber and burned.  Mightn't this provide an energy "boost"?

Edited to say, this is off-topic - my apologies!

On your thread-relevant question "Why city-lights?", I can only reiterate what I intimated above - it's because we like seeing our cities as places of light, not darkness.
To heck with the expense!

[alancalverd]

Alas, passenger jets generally fly too high to ingest many insects, and flying bugs just clog up the air intake of a piston engine without enriching the fuel burn. That's where millions of years of bat evolution  beat a hundred years of human thought: the bat can ingest, process, eliminate the waste, convert the protein into muscle (engine maintenance in flight!) and the fat into fuel, whilst doing all sorts of aerobatics and courting a mate, in pitch darkness, then settle down and make baby bats. The best I can manage after a night flight is to fall asleep in a taxi. 

[syhprum]

When I was at Uluru a few years ago and saw the milky way glowing in the sky and Venus casting sharp shadows I thought how wonderful it would be to have the wartime blackout back but when I drive on the crowded M25 at night my ideas change. 

[zx16]

I love driving in my car at night, with its dashboard softly glowing in multi-coloured dials, and the car's powerful headlights brilliantly illuminating the road ahead, as the car swiftly motors along, traversing a mile a minute! This is a glorious and intoxicating celebration of  Science! It's a dream come true, like in Jules Verne.

I mean, isn't it much better than being uncomfortably jolted slowly along over rutted roads in a leaky wooden 18th-century stagecoach, dragged by tired horses?