Classification of Traffic.

—The object of a classification is to obtain: (a) the maximum loads and average number of heavy loads per day. (b) The lighter loads, whether horse drawn or motor driven, iron, solid rubber or pneumatic tire, trailers, traction engines, animals, harnessed and unharnessed, and any other load which might affect the wear of the roadway. (c) Any other interesting data regarding the traffic, such as, local or through, cars belonging within or without the state or county, camping parties, and so on.

Maximum wheel loads are required, primarily, to see if state regulations regarding them are being complied with. In the Connecticut census it was found that a majority of trucks were loaded beyond their rated capacity and many of them beyond the legal maximum wheel loads.

Again heavy loads stress a pavement near to its elastic limit every time they pass over it. It is well established that any structural material when stressed near, to, or beyond the elastic limit will become fatigued with repeated stresses. The higher the stress the quicker they will fail by fatigue. It is not considered good practice to stress a material, especially a brittle material, repeatedly to a point half-way to its elastic limit. (The elastic limit is that stress that may be given to a body without producing a permanent set; below which it will return to its original shape upon the removal of the stress.) A walnut may not crack at the first blow but with a sufficient number of no harder repeated blows will crack. The higher the stresses the sooner the crack occurs. So it is important to know how many loads daily are stressing a pavement near to its elastic limit. From all the loads it has been the endeavor of road engineers to work out “importance factors” that will measure the relative damage done by the several classes. The Road Board of Great Britain has adopted the British ton as a unit and calculations are based on the traffic in tons per yard of width per year or per mile.[178]

The British Road Board Unit Weights are as follows:

Classification of Vehicles	Assumed Average Weight in Tons.
Ordinary cycles	0.09
Motor cycles	.13
Motor cars (including motor cabs and any other motor vehicles)	1.6
Motor vans (covered)	2.5
Motor omnibuses	6.0
Motor lorries (rubber tires)	6.0
Trailers to rubber tired lorries	5.0
Motor lorries (steel tires)	10.0
Trailers to steel tired lorries	5.0
Light tractors	5.0
Trailers to light tractors	5.0
Traction engines	12.0
Trailers to traction engines	8.0
Light vehicles (one horse)	0.4
Light vehicles (two or more horses)	0.6
Heavy vehicles (one horse)	1.25
Heavy vehicles (two or more horses)	2.5
Omnibuses (two or more horses)	3.0
Tram cars (electric, steam or horse, as the case may be)	....
Horses (led or ridden)	0.5
Cattle	0.3
Sheep and pigs	0.1
Hand carts and barrows	....
Horses drawing vehicles (to be calculated from number of vehicles)	0.5

The French unit of traffic is technically known as the “collar,” a draft animal harnessed to a wagon being counted as 1.0. The metric ton, 1000 kg., is also sometimes used. The French, feeling that the dead weight of a vehicle or animal did not truly measure its effect as to wear on a road surface, classified the traffic and assigned importance factors to the several classes. From 1882 to 1903 the classification consisted of: 1st, Trucks and farm wagons, loaded; 2d, Public vehicles designed for transporting passengers and their baggage; 3d, Light vehicles, such as private vehicles, and empty farm wagons; 4th, Larger animals, such as horses, mounted or not, mules, and large cattle; 5th, Small beasts, such as sheep, goats, and pigs. In 1903 motor vehicles were separately listed; they were divided into five classes: 1st, Metallic-tired automobiles, “which in general are heavily loaded, have a slow movement and produce the effect of wearing away the road surface”; 2d, Elastic tired automobiles licensed to make a speed of not more than 30 km. per hour; 3d, Automobiles whose speed was less than 30 km. per hour; 4th, Bicycles or velocipedes propelled by the feet of the rider; and 5th, Motor cycles, whether having two, three, or four wheels. The report of the second international road congress further states that “it is necessary to attribute to each element of the traffic an importance which belongs to it from the viewpoint of the destructive effect exercised on the road crust.” Consequently the numbers of vehicles or animals in the several classes were modified by multiplying them by importance factors arbitrarily assumed.

Importance	Classification factors
An animal harnessed to a loaded vehicle	1	collar
Loaded trucks and farm wagons	1
Public vehicles for transporting passengers	1
Light vehicles, and empty farm wagons		¹⁄₂
Harnessed horses to light vehicle or empty wagon		¹⁄₄
Mounted horses or with load on back		¹⁄₄
Unharnessed horses		¹⁄₅
Cattle		¹⁄₅
Small beasts (sheep, pigs, goats)		¹⁄₃₀
Automobiles with metallic tires, weight in metric tons times 1¹⁄₄ = No. of collars
Automobiles with elastic tires
Motor cycles		³⁄₁₀
Vehicles licensed to make a speed over 30 km. per hour	3
Vehicles licensed to make a speed of less than 30 km. per hour	1

From this it was possible to reduce all traffic to the unit “collar,” which was used as a comparative measure of the use of the several roadways. The tonnage was calculated by multiplying the numbers by average weights obtained by weighing a sufficiently large number of the units in each class.

Consideration was also made of the weight of the useful load as separate from the weight of the vehicle itself. Animals not harnessed were considered as a part of the useful load.

In Italy traffic censuses followed practically the same classification and methods as in France.

In the United States some of the states have used coefficients of reduction, or importance factors, while many others have contented themselves with a count of vehicles only.

In 1910 Maryland used the following:

1.	Ridden horse and one-horse vehicle	2
2.	Two-horse vehicles	4
3.	Three-horse vehicles	6
4.	Four-horse vehicles	8
5.	More than four-horse vehicles	12
6.	Motor cycles	2
7.	Motor runabouts	10
8.	Four- or five-seat touring cars	20
9.	Six- or seven-seat touring cars	40
10.	Motor trucks or drays	20

The New York State Highway Department took a census in 1909 in which the following classification and reduction coefficients were used:

Class of Traffic	Relative Weight
Horse-drawn traffic
Horses with vehicles	1
One-horse vehicle, light	2
One-horse vehicle, heavy	3
Two-horse vehicle, light	3
Two-horse vehicle, heavy	4
Three-horse vehicle, heavy	5
Four-horse vehicle, heavy	6
Motor vehicles
Motor cycles	1
Two-passenger cars	2
Three-passenger cars	3
Four-passenger cars	4
Five-passenger cars	6
Seven-passenger cars	7
Trucks, omnibuses, etc.	10
Miscellaneous
Traction engines	15
Two traction engines	30
Miscellaneous heavy traffic	5 upward

The Massachusetts Highway Commission, 1912 Report, say, “After all it is not numbers which tell the story, it is weight, and it is not weight alone, but the vehicle by which it is transported, whether by horses or by motor.... All these considerations are probably not so important on many road surfaces as the actual weight imposed upon the road per inch width of tire resting upon the road.” There was used in this census the following weights:

Motors	Tons
Runabouts	1.43
Touring cars	2.23
Trucks	6.25
Horse-drawn vehicles
One-horse, light	.36
One-horse, heavy	1.12
Two or more horses, light	.54
Two or more horses, heavy	2.46

James and Reeves, with the United States Bureau of Public Roads, recommend the ton-mile basis and give the following weights:

	Tons
One-horse wagon, loaded, 0.88; unloaded	0.28
Two-horse wagon, loaded, 1.57; unloaded	0.47
Four-horse wagon, loaded; 3.88; unloaded	0.54
Pleasure vehicles, one-horse, 0.28; two-horse	0.47
Rubber-tired pleasure vehicle	0.28
Saddle horse	0.50
Motor cycle	0.20
Excessively heavy vehicle	3.94
Motor, runabout, 1.68, touring car	2.00
Motor dray, loaded, 2.43; unloaded	1.23
Draught horses	0.50

In a traffic census taken by the Borough of Brooklyn, New York, the weights were reduced to traffic units per minute per foot width of roadway which was called density. By this rule, “the number of vehicles passing a given point in eight hours times the traffic unit divided by 8 times 60 times the width of the roadway equals the density.” The weights and traffic units used were:

Rubber-tired vehicles	Weight in tons		Traffic value
Large automobile trucks, loaded	9		5
Large automobile trucks, empty	4		4
Small automobile trucks, loaded	3		3
Small automobile trucks, empty	1	¹⁄₂	2
Pleasure automobiles	1	³⁄₄	1
Carriages	2		2
Steel-tired vehicles ranged in weight from 1 to 7¹⁄₂tons and in traffic value from 2 to 10.

A suggested form for a traffic census sheet presented by a committee appointed to study the question of traffic censuses to the New Jersey State Association of Roads is shown on [p. 245]. This sheet also bears, for the use of the office, blanks for the tabulation of the traffic by classes:

Kind of Vehicle		No.	Weight	Vehicle-Tons
Motor cycle		.....	0.25	..........
Light-horse, empty		.....	1.25	..........
Light-horse, loaded		.....	2.00	..........
Heavy two-horse, empty		.....	3.20	..........
Heavy two-horse, loaded		.....	6.00	..........
Light pleasure motor car		.....	1.50	..........
Heavy pleasure motor car		.....	2.50	..........
Light motor truck, empty		.....	1.00	..........
Light motor truck, loaded		.....	2.50	..........
Heavy motor truck, empty		.....	5.00	..........
Heavy motor truck, loaded		.....	10.50	..........
Specials:	10 tons	.....	.....	..........
	15 tons	.....	.....	..........
	Over 15 tons	.....	.....	..........
Total		.....	.....	..........
Tonnage per foot width of pavement			....................
Tonnage per foot width of roadway			....................

SUGGESTED FORM OF TRAFFIC CENSUS SHEET

Traffic Census Sheet County Number........ Station No....... County......
State Highway Department of New Jersey................................ 192...
............................................... Road at..........................................
Exact location................................................................................
Count taken..............from...........to.............from...........to..............

Time Count Was Taken					Motor Cycle	Light Horse		Heavy Horse		Pleasure Motor Cars		Light Motor Trucks		Heavy Motor Trucks		Specials			Street Cars	Hourly Totals
Time Count Was Taken					Motor Cycle	Empty	Loaded	Empty	Loaded	Light	Heavy	Empty	Loaded	Empty	Loaded	10- tons	15- tons	Over 15-ts.	Street Cars	Hourly Totals
6	a. m.	to	7	a. m.
7	a. m.	to	8	a. m.
8	a. m.	to	9	a. m.
9	a. m.	to	10	a. m.
10	a. m.	to	11	a. m.
12	noon	to	1	p. m.
1	p. m.	to	2	p. m.
.............
3	a. m.	to	4	a. m.
4	a. m.	to	5	a. m.
5	a. m.	to	6	a. m.
Total

Of above motor vehicles.........carried foreign licenses as follows.........
................................... Weather.....................................................
Type of pavement....................... Condition of pavement...................
Width of roadway........ Width of pavement........ Traffic.... Narrow ...
Tires................... Special............... Inspector..................................
Notes............................................ Checked by...............................