Aesthetic qualities result from the appropriate arrangement of visual design elements and are used to evaluate a visual composition. These design qualities are intangible; they are perceived qualities that arise from relationships of design elements.

Order is the arrangement of design elements so that each element has a proper place and function so the whole works together as a unit without confusion.

	This bridge has a little bit of everything...except order.
	The repetitive pier shapes and continuous girder depth give this bridge a sense of order.

Proportion is a method of creating a sense of order by assigning appropriate relative sizes to the various elements. The goal is appropriate proportions between the various parts of a structure: between its height, width, and depth; between solids and voids; between surfaces and openings; and between areas of sunlight and shadow.

The proportions of this bridge, the large depth at the abutments compared to the depth at the crown, give it a very slender appearance. Ottawa, Canada.

Proportion can suggest the order of significance of the elements or the role played by the elements in a structure\emdash their relative size classifies some as performing principal functions and others as attending to secondary functions. For example, a slender column suggests a light load-carrying function, whereas a thick column suggests the opposite.

There can also be proportional degrees of surface texture and color.

Rhythm is a method of creating a sense of order by repeating similar elements in, on or around a structure. When these elements create a natural flow which is satisfying to the eye, rhythm is created. It requires that the elements have some similarity of visual characteristics in addition to a modulated placement. In bridges, for example, major rhythms are created by the repetition of similar pier shapes. Minor rhythms may be created by the spacing of light poles, post spacing within a railing, or even the horizontal rustication on a pier.

The larger main span creates a variation in the major rhythm of the piers, while the light posts create a consistent minor rhythm. Severn River Bridge (US Naval Academy Bridge), Annapolis, MD.

Harmony means that elements of a design have visual similarity. The relationship must be complementary. If planes or lines in a design have more dissimilar characteristics than they have similar characteristics, they are not likely to be perceived as harmonious.

	The shapes of these piers are similar, and thus harmonious, even though their sizes are different. I-95, Cheverly, MD.
	The differences between the hexagonal elements of the retaining wall and the rectangular elements of the noise wall, differences which are reinforced by the color contrast, make this an inharmonious composition. I-295, Trenton, NJ.

Visual balance is the perceived equilibrium of design elements around an axis or focal point. Rather than a physical balance, it may refer to equilibrium of abstract elements of design, such as masses, visual weights or texture.

Freeway compositions work best when they are balanced about the median centerline. I-66, Virginia.

Contrast relieves the monotony of simple harmony by complementing the characteristics of some design elements with their opposites. This adds a heightened awareness of each other. Contrast often takes the form of dramatic differences in color or light and shadow.

A second principle of contrast is that of dominance, where one of two contrasting elements commands visual attention over the other. One becomes the feature and the other becomes the supporting background. A dominant theme is essential in organizing the design into a pleasing aesthetic experience.

Scale refers to the size relationship among various features of the highway and between the highway and its surroundings. Since most design concerns itself with things that are to be used by people, a connection exists between the human body and designed objects. We often refer to structures that respond to the size of the human form as having human scale.

Highways have a larger scale because they are built for vehicles moving at a high rates of speed. Highway elements such as piers or girders can be very large but appear "in scale" with the highway environment. Conflicts in scale become apparent when highway elements become part of a pedestrian environment or adjoin buildings. Ways must then be found to reduce the apparent size of the highway element so that it fits into the smaller scale environment.

	Highways are very large elements even compared to city buildings. I -5, San Diego, CA.
	The overwhelming size of a typical highway element becomes clear when it is inserted into a pedestrian environment. San Diego, CA, Airport Access Road.

What people perceive is not always what is there. Our vision is susceptible to manipulation and illusion. Designers can use illusion to improve the appearance of an element. For example, placing a series of vertical grooves on a column will make it appear thinner.

	Some common visual illusions that can assist with highway design.
	The slanted lines of the abutments create the illusion that the girder is longer, and thus thinner, than it really is. I-70, Denver, CO.

Unity provides the observer with a sense of wholeness. This is generated by some central or dominating perception in the composition. It encompasses the perfect application of all the other qualities, and it refers to the combined effects of all other aesthetic qualities applied simultaneously. Unity is the condition, or state, of full resolution of the site and project functions. It implies harmony where all of the elements are in accord, thus producing an undivided total effect.

The arch provides a central feature, reinforced by its strong color, that ties together both the man-made and natural features of this scene. I-70, Frederick, MD.