I was caught speeding at 37 mph on the A40 in London, where the speed limit changes from 40 mph to 30 mph. I was driving in the rightmost lane around midnight. The 30 mph speed limit sign on the central barrier next to my lane was graffitied over and obscured. I did not see the non-graffitied 30 mph sign on the left due to a vehicle blocking my view. I assumed the speed limit was still 40 mph based on the obscured sign and previous signs. TFL Streetcare has photo evidence that the 30 mph sign was graffitied/obscured before my alleged offense. TFL confirmed in writing that this sign is frequently targeted by vandals. I have a completely clean driving record and drive extremely cautiously with limiters on. I would have absolutely slowed to 30 mph if the signage was legible. I was offered a £100 speeding awareness course, which is difficult for me as a PhD student on a stipend. Given the obscured signage, my driving record, and willingness to follow limits, I am considering pleading not guilty.