Development densities and the mix of those developments have a key role to play in affecting both the level of travel and overall travel patterns. Higher densities of development may encourage shorter journeys and, thus, the use of walking and cycling. They may also help to make public transport more viable. In a similar manner, a better mix of developments can improve accessibility and hence reduce the need to travel.

Although neither terms are inextricably linked it is useful to consider them together. Development density refers to the number of people or jobs in a given area, whilst the development mix refers to how land uses are arranged in relation to each other. If common destinations are located close together, this type of mix is sometimes known as clustering.

The mechanisms through which density and mix have an impact on travel demand and travel patterns are:

• Accessibility – the number of potential destinations located within a geographical area tends to increase with population and employment density, reducing travel distances and the need for private travel. For example, in low-density areas a school may serve fifty square miles, requiring most students to travel by motor vehicle. In higher density areas, schools may serve just a few square miles, reducing average travelling distances and allowing more students to walk or cycle. Similarly, average travel distances for errands, commuting and business-to-business transactions can decline with density increases.
• Transport choice – Increased density tends to increase the numbers of transport options available in an area due to economics of scale. Higher density areas tend to have better pedestrian and bicycle facilities and better public transport service because increased demand makes them more cost-effective. As a result of these factors, higher density and clustered land use mix together tend to reduce per capita car ownership and use, and increased use of alternative modes.

Any impacts on travel demand and patterns arising from changes in the density and mix will depend upon: 1) the scale of the land use changes; 2) the design and type of changes, in terms of density and mix; 3) the speed with which the changes are affected.

It is also necessary to view changes in density and mix as being inextricably linked with a variety of other land use factors such as roadway connectivity and design, parking facility design, and building design, all of which will impact upon travel patterns.