ABSTRACT
This chapter provides guidance on the geometric design and spacing of traffic calming measures. Geometric designs of traffic calming measures are based primarily on desired speed at so-called slow points. Geometric designs should be based on dimensions of vehicles regularly found in traffic stream. For most typical designs, a single-unit truck is the "design vehicle," on assumption that it is largest vehicle commonly using the neighborhood streets being traffic calmed. The mini-traffic circle was pioneered in the United States by Seattle circa 1980. Seattle originally designed its traffic circles so that vehicles could pass to the right of center islands when making left turns. Roundabouts are distinguished from mini-traffic circles by larger radii, correspondingly higher design speeds and capacities, and splitter islands on all approaches, which slow traffic and discourage wrong-way movements. In designing a traffic circle, the desire to maximize vehicle deflection needs to be balanced against the possibility of placing the deflected vehicles too close to pedestrians.
