So blow horn or sound horn

When I first arrived to India and left the airport in New Delhi, I was going by car to the hotel. It was late night about 2-3 am. Dark roads, and trucks on highway. I read on back side of one of the trucks the following:
"Blow horn"
"Use dipper at night"
First time I saw it, it was very funny. I thought that it is a joke :) But then there were second, third, tenth trucks... And all of them had the same writing on the back. I didn't ask the driver about it at that night, because we both were very tired.
On the next day I moved to Chandigarh. Same writings... :) I was always smiling when I see this writings :)
At the end of 3rd week in Chandigarh, we got an idea to go to Shimla. Idea came on saturday evening at 5 pm. At 5:05 pm, we were on the road to Shimla :) 5-6 hours way to Shimla was full with trucks... and these writings... And now I started to understand that it is not a joke... I would say it like a part of driving culture in India :) If you are slow write it :) Something like that. In any case, such writing in any other european country would be a joke, but not in India... :)
At the end of 4th week I moved to Chennai and I was expecting that I will see same writings here... But I saw different:
"Sound horn"
It was very impressive... :)))