How deep is that trench?

We took a bus from Singapore (SG) to Kuala Lumpur (KL), which is pretty convenient, even the customs & immigration are not too difficult. The offices are on opposite ends of the causeway between SG & Johor Bahru, the most southern city in peninsular Malaysia. You simply get off the bus in each place, and breeze through customs, at least if you're gwai-lo / mat salleh (foreigner, white devil, etc. in Chinese / Bahasa Meleyu). I've only been searched once on this entire trip, which was in Narita, where I was actually searched 3x in about as many minutes. In any case, I simply walked through both, Rita, who's Malaysian, gets searched nearly every time.

We were seated very near the front, with only a middle aged Chinese woman in front of us, who promptly pulled down all the shades so we couldn't see out the front, and turned off all the AC in her section, then proceeded to cough and hack the whole way. I was irritated with her, and was waiting for her to fall asleep so I could go turn on all the AC again. Fortunately the AC on the bus worked very well, and eventually got too cold, something I thought was impossible in SEA. About half way through the trip, they stopped the bus for a piss stop, drink break, etc. She was the first one off the bus, and we followed slowly behind her. I was looking at the drinks & whatever on the little truck in front of the tandas (bathrooms). All of a sudden, I heard a yelp and a thud. This poor woman had walked right into a four foot deep concrete trench that was only 18" wide or so. I helped get her out of the trench and took a look at her injuries, which were slight, although she probably had some nice bruises on her legs judging by the abrasions on her pants. A silver bracelet had fallen off her arm, and one of the Malay guys working the truck jumped into the trench to get it for her. His head barely cleared the top of the trench and he needed an arm up to get back out. She was somewhat lucky in that had she fallen about two feet to her right, she would have almost certainly hit her head on the grating she was supposed to have walked across in the first place.