Well if the driver is parked and sleeping it is for heat, or air conditioning.
But other than maybe if it’s super cold and the driver has no way of plugging the truck in to insure it starting ,they really shouldn’t leave them run for extended periods of time.

Oh should add if it’s a delivery truck it could be for the power tail gate.

@SQUEEKY2 I have wondered this stuff, thank you!

I used to have a refrigeration unit on my truck that only worked when the engine was running. If I was doing just a short delivery I would leave it running to keep the product cold.

For refrigeration usually. When I checked in vendors with frozen or refrigerated items at my old job they would need to keep the truck running.

@SQUEEKY2 Thanks for your answers! I’m asking mainly about trucks on my street, not drivers sleeping in their trucks but the answers make sense.

It has to do with it being a diesel truck.

@SnipSnip Can you explain that further?

Amazon, UPS, and the like are only here for about 5 minutes. Other than that I’m not sure which trucks you mean.

@smudges On my street, trucks delivering furniture or household goods are often parked for 15–20 minutes, construction trucks of all kinds, utility trucks…..It can be a real Richard Scarry paradise some days!.

Ohhh..ok. Gotcha! I’ve always meant to research this but never have…I want to know if it’s better to turn my engine off for a longish wait at the drive-thru (bank, food, whatever) or would it take more gas to restart it. Maybe that has something to do with trucks and deliveries. It’s prolly one of those answers that everyone has their opinion on, rather a scientific definitive answer.

@janbb Diesel trucks need a period of time to warm up before they head out on the road pulling several tons of cargo down the interstate at 60+ MPH. Therefore, many truckers will keep the trucks running to keep them warm and to keep oil and coolant circulating the motor.
(https://www.bigrighq.com/why-do-truck-drivers-leave-their-trucks-idling-so-much/)

Maybe they don’t know how how long the delivery will take.

All the reasons above, plus I think it is a signal to cops that the truck is just standing not parked, and hopefully won’t get a ticket if they are illegally parked. Wouldn’t work for fire hydrants or dangerous things like being too close to a corner, but might for blocking a driveway or a double park.

More than a minute I assume it’s wasting fuel.

Her is a fun fact about diesel engines!
When idling, there is only a mere drop of fuel being injected into each cylinder, which isn’t enough to keep the engine at proper operating temperature (180 degrees). If the engine RPM is raised to a minimum of 1000 RPM, the engine will remain at 180 degrees, which is good.
So a diesel engine should not be idled!

What occurs when a Diesel engine is idled??
Since the engine is not at proper operating temperature, some of the injected fuel condenses on the cylinder walls where it forms a varnish (gunk, if you will) that causes the piston rings to stick/seize, increasing compression blow-by and thus shortening the life of the engine.

I agree with @JLeslie Maybe the driver thinks if the car is running that they can’t get a ticket.

I am unsure if there are any regulations related to this set by the Department of Transportation / FMCSA Federal Motor Carrier Safety Administration
If it is diesel then the fuel consumption is minimal and they typically leave it running.
Adding to noise and pollution is a whole other consideration.

I remember years ago seeing a TV spot about some company in Europe, I think it was Germany, had very strict rules about how long their truck drivers could idol. It wasn’t a big rig, more like a delivery sized truck. It was to conserve gas and pollution.

In addition to the answers above, delivery drivers always get stopped by people inside and trapped in a 20 minute conversation about worthless small talk.

Some people love to claim that work is important and being an efficient and reliable employee is key to success, yet love wasting hours throughout the day talking about conspiracy theories and liberals.

And to be clear about my answer, please note I USED to have a delivery truck that required the engine to run for the reefer to work. Upgrades in technology have made it so the reefers can run on their own with or without the engine running. And I didn’t like leaving my truck running. A truck full of beer and wine tends to drive away in some areas without me in it if I leave it running and unattended.

Once when I was staying in a motel, a big old truck pulled into the small parking area (right in front of the rooms) and the driver left the engine running. It was one of those deep-throated, rumbling engines that were enormous dinosaurs in an earlier incarnation. The engine idled for three hours, like all evening, until I couldn’t stand it any more. I called the front desk and asked if they could have it shut off. They said, “Oh, is it bothering you?” like I was just unusually sensitive. And then it stopped.

Why was that normal?

@Jeruba I stated why it would be left running,but for a driver to pull in front of a motel and leave it running is just rude.

@SQUEEKY2 Are you saying 3 hours is normal except for the noise being rude?