Restriction signs and depth indicators

Following on from the previous article "Supplementary plates for warning and restriction signs", this article looks at restriction signs and depth indicators, which needs to be converted to metric-only.  This article gives examples of metric-only signs for height restrictions, width restrictions, and length restrictions signs, signs warning of restrictions ahead, and correcting the symbol on the already metric weight restriction signs.  In addition to this, an example of a metric-only depth indicator is shown.

Height restriction signs

Metric-only height
restriction.

Converted height restriction and distance
on non-primary / local route sign.

Converted height restriction and
distance on primary route sign.

A possible replacement of "avoiding
low bridge" with a symbolic equivalent.

Metric-only height restrictions need to be authorised as soon as possible. There is no need to have the imperial measure on the sign as it only adds clutter.

I also believe the mandatory height restriction sign is sufficient for all (or almost all) purposes regarding maximum available height, and should definitely be used on all non-arch bridges. As expected, vehicles greater than the specified height are forbidden from passing under the bridge.  The majority of height restriction signs in the UK are the height restriction roundels anyway.

Naturally the height restriction signs will need to be replaced on the bridges, tunnel entrances, or wherever only vehicles up to a certain height are permitted. As well as this, the height restriction signs need to be converted on the bridges, and also on direction signs and signs showing the distance and/or direction to the restriction, and any signs which incorporate height restrictions in general. All these should be converted to metric only, not the cluttered dual-unit signs. The sooner we have metric-only height restrictions the better for road safety and clarity.

Direction signs often show how to reach a place while avoiding a low bridge with "avoiding low bridge" directly below the place name. An improvement I would suggest is that "avoiding low bridge" gets replaced with a diversionary symbol to the right of the place name instead (for example a gold rhombus with a black border, or a black ring in a gold border. Road signs need to be as language independent as possible.

Maximum headroom warning sign (included for completeness)

Maximum available headroom
warning sign.

The maximum headroom warning sign, which warns road users about the maximum headroom available, also needs to have the metric-version authorised alone without the need for an accompanying imperial sign.

There is perhaps a case for retaining the maximum headroom warning sign only for arch bridges.

Arch bridge warning sign (included for completeness)

Arch bridge warning sign.

There is also an arch bridge warning sign, of which the metric version also needs to be authorised alone, and which there is perhaps also a case for retaining.

The arch bridge warning sign is not used on the arch bridge itself, but is used to warn drivers approaching an arch bridge.

Why mandatory restriction roundels could be sufficient for height limits

The question remains why there are three different signs are there for height restrictions / available headroom (one restriction and two warning signs).  My opinion is that the circular mandatory restriction sign is sufficient for all (or almost all) purposes related to height safety, because it means that there is only one sign to remember for height restrictions.  Indeed, some triangular headroom warning signs have already been replaced by height restriction roundels on non-arch bridges.

In addition, one can include a supplementary plate (symbolic or otherwise) to the circular height restriction signs to show exceptions if any are applicable, which could eliminate the need for triangular warning available headroom signs and triangular arch bridge warning signs in most circumstances.

However, it is much more important that the signs for height restriction purposes are solely metric.

Safe heights under power cables

Overhead cable ahead,
safe height 5.0m.

For safe heights under a power cable, shown to the right is a suggested replacement with a wholly symbolic supplementary plate. Metrication gives us an opportunity to improve this type of sign by making the supplementary plate language independent and symbolic. So instead of having a supplementary plate which has in words "Safe height" followed by the measurement, the supplementary plate should be symbolic and mean safe height, and an example of such a sign with a symbolic plate meaning "Safe height 5.0m" is shown on the right.

In general, wholly symbolic supplementary plates should be used where possible (and where not already done so), because it is much quicker to glance at the symbol than to read the supplementary plate.

Width restriction signs

Examples of converted width restriction signs are shown. Three possible designs are shown below:

Three possible designs for a 2.0m width restriction sign.

The design on the right would of course only apply for widths that are a whole number of metres (for example 2m, 3m, etc.) if this was to be used. The design on the left or centre would be used in the general case (1.9m, 2.2m, etc), and can be used for for all cases.

Length restriction signs

Metric-only length restriction signs must be authorised.  Examples are shown below:

15m length restriction (left); 10m length restriction except for access (right).

Weight restriction signs

Although weight restriction signs are already metric (in tonnes), they still use the incorrect symbol "T" (which is the symbol for teslas, which is the measure for magnetic flux induction). These need to be corrected to use the correct symbol for tonnes "t".  There should also be no superfluous abbreviations either (like "m.g.w.") where relevant.

Example corrected signs are shown below:

7.5t general weight limit (left); 16t LGV weight limit except for access (centre);
4t axle weight limit (right).

Depth indicators

Depth indicators, referred to by the DfT as depth gauges, are normally added at fords (where a road crosses a waterway such as a river or stream), or where flooding is a problem.  Metric only depth gauges must be authorised and used to replace the imperial-only gauge (and ideally the dual-unit gauge too), and actually use the correct symbol "m" for metres, not "M" as shown in the Traffic Signs Manual page 45. An example metric-only depth gauge is shown to the right.

Although this is not strictly related to metrication, an alternative solution to fords is to simply have low bridge crossings, or bridges over the fords and/or flood prone areas, and eliminate the need to drive in the water altogether.  Roads which pass through fords are usually found in remote areas, rural areas, or both.

Signs warning of restrictions ahead

As the UKMA has already demonstrated here, current dual-unit warning signs are cluttered, and there are far too many superfluous words on such signs as well, or the signs themselves are wholly wordy, and gave examples of why the metric-only equivalent is clearer. 

This article also includes some brief examples, which also shows the suggested improvements, and all the examples illustrate how metric signs look cheaper to make and maintain, and how much easier to read at a glance they are. The examples are shown below:

Height restriction a certain distance ahead sign:
Before conversion (left); After conversion (right).

Weight limit to the right sign:
Before conversion (left); After conversion (right).

LGV weight limit a certain distance to the left sign:
Before conversion (left); After conversion (right).

Through traffic prohibition / restriction, a certain distance ahead:
Before (left); After (centre); After alternative conversion (right).
The nature of the prohibition / restriction can vary.

Replacement of wordy height restriction sign with suggested diversion:
Before replacement (left); After (centre); After alternative conversion (right).
The height restriction may not necessarily be ahead, it could be on the left.

From these examples, we can see that the converted metric-only restriction warning signs are clearer, easier to read, cheaper to manufacture and maintain.  The metric-only versions of these restriction warning signs demonstrated here are easy to read at a glance, and are language independent, all of which helps in safety. Where relevant, schematic diagrams can also be used to show a diversionary route.

A comment on the DfT's dual-unit restriction sign proposals

The Department for Transport (DfT) propose to make dual-unit (imperial and metric) signs mandatory for height and width restrictions on all signs within the next four years, as part of the Traffic Signs Regulations and General Directions (TSRGD) 2010. It is a first step, but does not go far enough. We need mandatory metric restriction signs, and fully metric signs (including distances and speed limits) which need to be planned for as soon as possible, and we need to get rid of all wordy signs and replace with the symbolic equivalents at the same time.