# Indication of A and B numbers in Britain

Last updated 11-8-2019, fixed links and tables 9-3-2024

# A and B numbers on signs

## Introduction

A and B numbers are in principle signposted in green (primary routes) or white (other routes).

Typical advance direcctional sign with primary and other A route

Primary route sign at the junction

In many cases, especially in dense urban areas, they are missing. This page gives statistics based on some thousands of observations.

# Rules and method

## Data points and categories

One data point is the indication or absence of an A or B number coming from one direction.

Random samples were taken of junctions between A and B roads in the United Kingdom. A large number of cases were categorised in the following ways:

Thus multiplying all possibilties we potentially have 408 slices:

From A to A: 2*2*2*2*(4*3+3*1)=240 since Northern Ireland does not have 4-digit numbers

From B to A: 1*2*2*2*(4*3+3*1)=120 since from B to A can never be the same number

From A to B: 1*2*2*1*1*4=16 - from A to B can never be the same number, there are (virtually) no primary B routes and the number of digits of B numbers does not matter

From B to B: 2*2*2*1*1*4=32 - here we can have the same number

Therefore even with a sample of several thousand there are many very small slices. To illustrate this, consider the fictitious situation below:

The A12 is a primary route running east-west, and the A11 is non-primary and joins the A12 to the east.

There are 8 different cases here:

Note: if the A11 continues as a non-primary route after it branches off from the A12 further east, it is considered non-primary also on the common section.

About 480 junctions were analysed this way giving almost 4000 data points. Data was recorded per case: e.g. if there are four possible turns to a different number at a junction and three of these are indicated, then indicated=3 and total=4 are recorded, not each individual turn separately.

Google Streetview was used for all junctions.

## Some rules

When a route follows along with another route, this is generally indicated between brackets in the United Kingdom. Example:

B4161 multiplex with A454 to the right between brackets - A4039 is an indirect reference

It should be noted that brackets are also very often used inappropriately: in the situation below, the B4484 is straight ahead and continues via the A4124 to the left.

B4484 multiplex with A4124 between brackets

This is ignored and any number indicated with or without brackets is counted as a success.

2. Visible signs

This sign is mostly aimed at traffic coming from the side road but is visible to drivers on the A4039.

In many cases road numbers only appear on advance directional signs, not on signs at the junction (if they exist at all), but these are counted as indicated.

3. Straight vs. turn

A maneuver is considered to be straight on if a vertical arrow on a sign or a lane indication indicates straight on, even if the angle is not exactly straight.

4. Bushes

Some signs are partly behind bushes in Streetview, this is ignored and road number indications visible on any pano are counted as success.

(See picture above - A4039 is behind bushes)

# From A to A routes

Most observations are for this case. Below are aggregated tables for the entire United Kingdom and the four countries.

## Some more aggregations

Each of the criteria is aggregated below.

The only significant difference is that primary routes are better signposted than other routes, as could be expected.

Normally the number of digits gives some indication of the importance of a route. Therefore it could be expected that the number of digits should be negatively correlated to the success percentage, but that does not seem to be the case, except that 4-digit numbers appear significantly less than higher numbers.

# From B to A routes

Given the lower number of observations, this is not split into countries.

# From A to B routes

This is also only analysed for the entire United Kingdom.

# From B to B routes

The table below shows aggreated data for the A to B and B to B cases combined.

Here we do see that multiplex numbers are much less signposted than single numbers.

Marcel Monterie

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