Section 4: InterchangesAnchor: #i1002969
This section addresses the following topics related to interchange design:Anchor: #i1003051
Types of Interchange
Interchanges are intersections that merit separate consideration because of complexity. The primary feature of an interchange is vertical grade separation of the intersecting routes to increase safety and efficiency. Grade separation is achieved using a series of ramps and bridges to accommodate the various directional movements.
Within each interchange type there are numerous variations in the ramp placement and configuration related to availability of right-of-way and traffic volumes.Anchor: #i1003068
Interchanges occupy deceptively large land areas. The bridges, along with their approach embankments, dominate the visual field. Attention to aesthetic design can enhance the appearance of interchanges.
Figure 5-4. The character of a large interchange can be enhanced with attention to the colors and details of its elements.
Because of the sinuous architectural qualities of interchanges they tend to be the primary focus of landscape and aesthetics design concerns. There are several good reasons for this:
- Interchanges have a high level of visual interest because of the structures and landforms involved.
- Interchanges mark entrances to adjacent communities and business properties and therefore strongly influence first impressions of that community.
- Because traffic is slower in the entrance and exit ramps of the interchange, architectural detail is more visible and more aesthetically important.
- TxDOT has full control over the design of the interchange.
- Interchanges are usually associated with freeways in heavily urbanized areas. Because of their size they dominate the visual landscape even in areas with large buildings.
Because the interchange is so important to the overall perception of an urban freeway and adjacent communities, it requires careful attention to landscape and aesthetics design properties. The following considerations will affect interchange landscape and aesthetics design decisions:
Pedestrian circulation and balanced design detail also affect interchange aesthetic design decisions.Anchor: #i1003177
Safety must be the primary consideration in the design of the interchange.
- Maintaining sight distances and the clear communication of traffic patterns is a paramount concern.
- Design elements, such as night lighting, must complement the structures and not interfere with operational requirements of the interchange.
- Design elements should not have reflective surfaces or be placed so that they cause visual interference.
Because of its size and the amount of open space occupied by an interchange, it is usually an integral part of the area or regional drainage network. Landscape and aesthetics design decisions must not adversely effect the drainage pattern.
- Plant materials should be selected and placed so that mature specimens will not interfere with the function of drainage channels or structures.
- Drainage facilities and channels should be incorporated into the overall aesthetic design scheme. Wetlands, detention, and retention basins can be visual assets if designed as part of the landscape and aesthetics treatment.
Planting in interchanges should be done as part of a comprehensive landscape and aesthetics plan. The design of the planting must be done so that it achieves the aesthetic goals for the specific corridor and facilitates maintenance of the interchange.
- Planting is most effective in areas of low slope. Planting on the slopes is difficult to maintain and will shade out grass cover, which leads to erosion.
- Planting is most effective when placed in the driver’s line of sight and where the background is either sky or light colored structures.
Design solutions must be sensitive to deeply shaded areas and areas that are difficult to access (see Figure 5-5).
- Bridges and tall embankments will shade areas of an interchange, making the establishment of a vegetative cover nearly impossible. These areas should be either eliminated structurally using walls, or surfaced with an appropriate non-living material.
- Shaded areas tend to collect debris, attract graffiti, and are sometimes occupied by transients. These areas should be eliminated structurally if possible. When this is not possible the views should remain open to allow visual policing.
Figure 5-5. A paved surface is a better solution than plants where shade from structures prevents vegetation establishment.Anchor: #i1003276
Gore areas between ramps and weaving lanes often contain crash attenuation barriers that are not particularly attractive. The triangular area between the diverging lanes tends to accumulate trash. Design actions appropriate in these areas are:
- use colors on pavement that contrast sharply with the driving lane pavement
- avoid rough textures that will trap and hold trash and debris
- group signs to provide a uniform horizon even if the signs are of different dimensions; such groupings contribute a sense of visual order (see Figures 4-61 and 4-62)
Pedestrian access through interchanges is a major consideration in urban centers.
The primary consideration in providing for pedestrian crossings at interchanges is to direct the pedestrian to crossing points that will:
- provide full view of the approaching traffic and the pedestrian
- minimize the distance the pedestrian is required to cross
- place crossings at points where traffic speeds may be reduced, such as entrance and exit ramps
- limit the traffic to a single direction if possible
- provide pedestrian refuge where multiple lanes must be crossed
- where possible, provide physical barriers between vehicles and pedestrians