Investigation into the operation and safety of freeway auxiliary lanes: Towards uniform design guidelines
Freeways operate closest to the purest form of uninterrupted traffic flow. There is no constant interruption to traffic flow (such as stop sign and traffic signal), and access is limited to ramp locations. Therefore, at freeways, bottlenecks, if exist, are usually caused by merging and diverging traffic at ramp locations. Bottleneck removal at freeway ramp areas can reduce average freeway commuter delay. Currently, engineers may add auxiliary lanes at ramp areas to mitigate or remove bottlenecks. ^ The American Association of State Highway Transportation Officials (AASHTO) publication A Policy on Geometric Design of Highways and Streets, commonly known as the "Green Book", defines an auxiliary lane as "the portion of the roadway adjoining the traveled way for speed change, turning, turning storage, weaving, truck climbing, and other purposes supplementary to through-traffic movement". In freeway design, an auxiliary lane typically refers to either the added lane between an upstream on-ramp and a downstream off-ramp, the acceleration lane immediately downstream of an isolated on-ramp, or the deceleration lane immediately upstream of an off-ramp. ^ While auxiliary lanes are widely used in urban freeway interchanges, broader understanding is necessary for the design and impacts of auxiliary lanes. The objective of this research is to first analyze the operational and safety impacts of adding auxiliary lanes, and then to develop guidelines on when and how auxiliary lanes should be implemented. In addition, look-up charts and tables have been developed to provide better quantitative understanding on how auxiliary lane improves operations and safety at freeway ramp areas. These guidelines and look-up tables developed in this dissertation help decision makers to better understand the expected benefits of auxiliary-lane use, including how auxiliary lanes address both congestion concerns in the proximity of freeway ramps. ^ For the operational impacts of auxiliary lanes, the study results showed that, after adding an auxiliary lane, the density reduced and level of service improved. At weaving segments, adding an auxiliary lane reduces density by 1.6 to 19.5 pc/mi/ln. At on-ramp junctions, after the addition of an auxiliary lane, the density in the merge influence area reduces by 3.0 to 9.4 pc/mi/ln. At off-ramp junctions, after the addition of an auxiliary lane, the density in the diverge influence area reduces by 4.5 to 13.5 pc/mi/h for LD=500 ft, and up to 13.5 pc/mi/ln. ^ For the safety impacts of auxiliary lane, the study results showed that, on-ramp junctions with auxiliary lanes have significantly lower average crash rate (in terms of number of crashes per MEV). However, there is no significant change in the average crash severity (i.e., portion of crashes that are fatal or result in injuries). With regards to crash types, sites with auxiliary lanes are observed to have significantly lower proportion of rear-end crashes and higher proportion of objected related crashes. In addition, the results also showed that, that crash frequency at on-ramp junctions with auxiliary lanes is negatively influenced by the length of the auxiliary lane, the percentage of heavy vehicles on the freeway. Nonetheless, it is positively influenced by the number of lanes on the freeway and the average daily traffic per lane on the freeway.^
Wang, Yubian, "Investigation into the operation and safety of freeway auxiliary lanes: Towards uniform design guidelines" (2013). ETD Collection for University of Texas, El Paso. AAI3597263.