Unconventional Left-Turn Alternatives for Urban and Suburban Arterials - Part One

ITE JOURNAL / SEPTEMBER 1998 MANY URBAN AND SUBURBAN arterials are congested with little immediate hope of relief. Access management and better coordination between land use and transportation offer long-term hope for developing areas but little short-term promise for developed areas. Transportation engineers in many places have done as much as they can with actuated signals, signal systems, multiple left-turn lanes, right-turn lanes and other conventional measures. Good parallel streets to create one-way pairs rarely exist outside downtown areas. Intelligent transportation system efforts have concentrated on freeways, and public transportation will require shifts in land use before it provides major relief. Widening arterials, building overpasses and flyovers, upgrading arterial intersections to interchanges and building bypasses are expensive and disruptive. To help treat this impasse, this two-part feature, which will conclude in ITE Journal on the Web in November 1998, offers seven unconventional alternatives which engineers may wish to consider for their urban and suburban arterials. The alternatives focus on treating left turns to and from arterials, which cause many of the operational problems on those facilities. Three of these alternatives have been used to a large extent in at least one state successfully for years. The other four newer alternatives are extensions of the older unconventional alternatives. The main purpose of this feature is to entice engineers into considering one or more of these alternatives during feasibility studies and functional designs. None of the alternatives is a universal solution and for many arterials none of the alternatives will work, but for some arterials the alternatives are promising. Little is wasted analyzing an extra alternative; much is wasted ignoring a better alternative. The unconventional alternatives share two major principles. First, the emphasis is on reducing delay to through vehicles. Serving through vehicles is the main purpose of the “arterial” functional class. Second, the unconventional alternatives try to reduce the number of conflict points at intersections and separate the conflict points that remain. This usually means reducing the number of phases at signals from four (assuming no overlaps) to two which reduces delay for through traffic and promotes progression along the arterial. This also usually means fewer threats to drivers which promotes safety. Six of the unconventional alternatives reduce the number of conflict points by rerouting some left turns and two reroute cross-street through movements. Agencies must have sufficient will, political backing, public relations resources, and, at times, enforcement resources to choose such an alternative and make it work. If your drivers or politicians have resisted past conventional measures, your agency probably should not attempt an unconventional alternative. By their nature as unconventional solutions, and by rerouting certain movements, the alternatives presented here all have the potential to cause more driver confusion than conventional arterials. However, newness is not a sufficient reason to ignore an otherwise superior alternative. In addition, the three older unconventional alternatives have shown that agencies can mitigate the confusion inherent in rerouting certain movements. In all three cases, including one state with an overwhelming amount of tourist traffic, the agencies have developed legible and understandable traffic control devices to guide drivers through the alternatives. The agencies also have found that driver expectancies are best met if they use the alternatives at several intersections or on a whole section of arterial. This feature does not provide quantitative guidelines for the alternatives. The unconventional alternatives, where the number of unprotected conflicting movements has been reduced, are theoretically safer than conventional arterials, but convincing collision data do not exist. The four new alternaUnconventional Left-Turn Alternatives for Urban and Suburban Arterials—Part One