That technically leaves open a different interpretation. There's a natural desire to fill in the missing words to make this a sentence. The alternative to "passing this vehicle" is to add "a" to make it "a passing vehicle".
There's a general message here. Firstly when signs become longer there's a real temptation for them to be read as sentences, and hence the emerging temptation to fill in any missing gaps, which might not necessarily be the right ones or in the right place. Secondly
This sign also really matters, it's a potential life saver prompt. Between 1986 and 2011, 439 cyclist were killed in traffic accidents in Grater London. Over a two week period in November 2013 six cyclists died from from cycling accidents in Greater London. That brought the total to for the year to 14, of which nine involved heavy goods vehicles. That also needs to be set in the context of the number of London cycling journeys almost doubling over the preceding 10 years. That appears to show a general trend over the longer term of reduced casualties per cyclist per kilometer per year. A potential life saver sign no less.
