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:

Sources

In most cases Google Streetview was used, sometimes also Mapillary and various other pictures.

Multiplex numbers

Routes are assumed to continue along other routes of at least the same class. Thus for example the A44 in Germany, which consists of many separate sections, is not expected to be signposted through all the gaps, but the B327 follows the B421 and B50 in this situation (and it is actually signposted that way). An indirect reference (e.g. between brackets, in another colour shield or in a dashed line) is allowed.

Example in the United Kingdom:

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

Visible signs

A sign that is aimed at traffic coming from the other side only and would only be visible to a driver looking back is not counted.

Using this principle, at roundabouts without advance directional signs, only signs to the left and straight are taken into account, as a driver can not be expected to contiue on the roundabout to see signs on the other side.

In general all signs visible to the driver in the given direction are counted, including signs aimed at a crossing road. It suffices that only some of the signs contain the road number (e.g. only advance directional signs, or only at the junction), even if according to general guidelines all signs need to show the number.

However, a road indicated only in one direction is considered missing in the other direction. This is common in Switzerland: 

Picture from 2006: H2 is only indicated in one direction, missing towards the right. In addition, the wrong shield is used. Both errors were corrected around 2010.

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

At every turn within an interchange / junction, the number must be indicated. In this example, the number is indicated at the exit but then not on the signs at the end of the ramp.

Neither the B20 nor the B21 are considered to be indicated from the west.

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.

Errors and additional numbers

Wrong numbers are ignored, but for changes a 10 year grace period is applied. However, it is not known (or checked) in all cases when numbers changed. Additional indirect references are ignored, even in case the wrong style is used in a country where indirect references are distinguished from direct references.

Road numbers indicated in the wrong colour, shield etc. are considered correct unless they could refer to the wrong road. For example, suppose that at the junction between the A1 and B1 the number A1 indicated in the style of B numbers, then this would be considered an error, i.e. counted as if the A1 were not indicated for the given maneuver.

Marcel Monterie