Is speed a variable? [closed]

This is a legal sentence:

London traffic moves at a speed of 11 mph.

Why can't we remove "a" and "of"?

London traffic moves at speed 11 mph.

We may say that "speed" is a variable name and "11" is the value, then a variable name doesn't need an article before as in the question.

Edit: A sentence above is a simplified example (academic writing) for something as:

..X moves at a speed of v..

Edit2: Also we may say that:

a. speed is a variable, because in London a speed is 11 mph, and Paris is 12mph, etc.. .

b. speed is a constant variable with a value 11 mph.

We may say that "speed" is a variable name and "11" is the value, then a variable name doesn't need an article before as in the question.

But I often see 'the radius r', 'a distance d'.

"The radius 'r'" and 'a distance d' are shortened versions of

"The radius, which, for the sake of brevity, we will call 'r'/which, for convenience, is labelled with the random letter 'r'"

The shortened form is suitable for questions in which the maximum information is conveyed in the minimum words and the literary style is not the paramount concern.

This is not at all the same as "London traffic moves at a speed of 11 mph.", which is (i) quite specific (ii) not a mathematical problem and (iii) the speed is not at all variable - you yourself have stated that it is 11mph.

It is possible to use "at speed 11mph" but only in a mathematical problem, e.g

A lorry has a mass of 20 tonnes. The bend in the road has a radius of 6 metres. At speed 11mph it may safely turn the corner; at speed 12mph it overturns. Calculate its centre of gravity.