According to the HSE guidance on non-verbal auditory warnings

I suspect that is a good pace to start. There would never be a set limit, because it would depend on how noisy your workplace was.

Depending on your work, 'out of hours' tends to be quiet, so no horns or alarms, certainly in or near most towns and cities. 

There are also alternative noises, that are less intrusive, but still warn.

Perhaps also think of large convex mirrors on corners of structures and I.R. light beams with flashing lights. 

Also be aware that not all people can hear, some suffer from photo sensitive epilepsy and some people are just plain daft. 

Is this a permament or temporary site?

What time?

Which day?

Vehicle type?

Your permitted or agreed working hours?

Your permitted or agreed noise emissions?

Good morning,

Many thanks for all your input and apologies for my late reply. The site has now opted for white noise reversing alarms where the units caan be reduced to 70dba. After consultations with the neighbours this appears to be an acceptable compromise.

Kind regards

Hope your site is not close to a main road where typical noise levels can range from 68dB (car on quiet tyres) to 105dB tipper waggon which would drown out any benefit of such a reversing alarm