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Definition of The Road Traffic Scale.

The concept of a Road Traffic Scale came about as I returned home from spending Christmas & New Year with my girlfriend. The journey was a regular one and would always be followed by the customary phone call to my girlfriend to say I had arrived safely. I would then be asked, "How was the traffic?" and I would reply with some token answer of a nondescript nature.
So, as I was driving down the M1, I made an effort to observe the traffic and take a mental note of the how heavy the traffic was. But it struck me that whatever words I decided to use to describe the flow of traffic, no-one would really know what they meant. This is because there is no well-defined meaning behind words like 'light', 'heavy' or 'congested'.
I realised that a simple scale that established a consistent set of terms would make describing traffic flow far more informative. What was needed was something akin to the Beaufort Wind Scale, a scale that is easy to use, such that anyone could make a reading from basic observations.

Why the Need for a Scale?

Traffic reports regularly contain phrases that describe traffic flow; e.g. 'heavily congested', 'static', 'solid'. However, reporters use a range of phrases to describe the same conditions. This makes it difficult to distinguish the meaning of the report and gauge which roads are worst hit by congestion. Equally as confusing: words such as 'congested' are over-used and are applied to traffic flow of varying severity.
What we currently have is a many-to-many mapping from traffic severity to descriptive phrase.

The development of a Road Traffic Scale will establish a clear reference for describing traffic flow, useful to both reporters and drivers.
The scale will allow reporters to use phrases consistently, which will help drivers to better understand the traffic report by knowing the definition of each phrase.
The Road Traffic Scale mimics the Beaufort Wind Scale. It defines a table of distinct classifications for traffic flow at increasing severity. Each classification contains a set of characteristics and maps those characteristics to a common phrase and numeric index. This gives the additional advantage that, like the Beaufort Wind Scale, a layman can use the scale to classify traffic flow by simply observing the traffic around him or her.

Rules for Developing the Scale

Before defining the scale, we must have a clear understanding of its objectives.

The scale must be appropriate for all types of road; from motorway to single-track lane.
Thus, when describing traffic severity we should refrain from mentioning multiple lanes, the ease of overtaking and absolute speed. All of these factors would relate to a specific type of road, thereby upsetting the generic nature of the scale.

The scale must also be appropriate for all levels of traffic volume; from busy cities to sparsely populated rural regions.
Therefore, the scale must extend from an absolute zero through an exhaustive range of circumstances - no matter how rare the extremities of the scale may be.

The scale will only attempt to grade traffic in free flow - i.e. traffic moving along a road and not traffic waiting at a junction.
It is accepted that on the approach to junctions, traffic will come under greater 'stress', which causes vehicles to slow down and stop. These situations should be considered anomalies in the regular flow of traffic.
An exception to this is where a queue of traffic at a junction tails-back sufficiently enough to affect the flow of traffic in what would otherwise be a place of regular flow.

Scale Definition

Severity Classification Characteristics
0 Empty No vehicles are using the road.
1 Clear Vehicles are infrequent; the road is clear enough for each vehicle to pass through the area without encountering another driver.
2 Quiet Few vehicles are using the road; the road is mostly clear; drivers are unhampered by other vehicles.
3 Light Several vehicles are using the road; no traffic is held-up; space between vehicles is ample; clear patches of road are frequent.
4 Fine Some traffic is held-up but vehicles travel at or around the speed limit; clear patches of road are infrequent; small queues occasionally form, but are not prolonged.
5 Busy Significant queues force traffic to be held-up at reduced speeds; there are no clear patches of road.
6 Heavy Traffic fills all available space on the road; large queues hold up traffic at speeds well below the speed limit; vehicles bunch up, reducing the gap between vehicles.
7 Mildly Congested Vehicles travel at much reduced speeds; traffic may occasionally come to a brief halt.
8 Congested Traffic is start-stop; vehicles crawl at slow speeds; vehicles stop frequently but only briefly.
9 Heavily Congested Most vehicles are stationary; when moving, traffic advances small distances before grinding back to a halt; vehicles remain stationary for several seconds at a time, but short enough for most drivers to hold the car using the foot-brake.
10 Solid Traffic barely moves; when moving, traffic creeps forward short distances; vehicles remain stationary for long periods, long enough for drivers to use their hand brakes.
11 Static Vehicles remain stationary for several minutes at a time; some drivers switch their engines off.
12 Gridlock Engines are switched off; all vehicles within the area are stationary for half-an-hour or more; vehicles become abandoned.

Notes on Using the Scale

The scale should be applied to traffic flow in one direction.
e.g. traffic can be (and often is) clear in one direction but heavily congested in the other.

Be aware that some characteristics overlap two or more classifications.
For example: if traffic is 'start-stop' it does not necessarily indicate 'Heavily Congested' (grade 9). Grade 10 also denotes 'start-stop' traffic, as does grade 8 - albeit more free moving then grade 9.
Try to identify and use at least two features of traffic flow to establish the severity.

Use decimal-points to refine your decision.
Do not feel constrained to the 13 marks on the scale. When you have identified the relevant characteristics of the flow of traffic, compare the severity of each feature against the scale. Use fractions of a grade to increase accuracy and assign a value that best represents all the characteristics.

When describing a trouble-spot, remember that the description of traffic flow applies to a whole area, not personal experience.
Some drivers describe a hold-up on a motorway because they were prevented from doing 90 m.p.h. and, in their frustration, do not notice that most of the traffic is flowing quite freely at 70 m.p.h. When analysing traffic from within the traffic, do remember to look around and use observations from the perspective of all vehicles in your vicinity.

When describing a complete journey you will normally need to make multiple references to the scale.
Do not try to average out numeric values as the meaning will become lost.
For example: if for most of the journey the traffic was fine (4.0) except for an area of roadworks, where the traffic became congested (8.0); to quote a mean value of 6.0 (heavy) gives no indication to the fact that most of the journey was unhampered nor to the severity of the delay at the roadworks.
Road Narrows First drafted on the 6th January 2004. Revised in May 2004 and again in August 2005.
The Road Traffic Scale by Stephen Battey is licensed under a Creative Commons Attribution-Share Alike 2.0 UK: England & Wales License.
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