Auxiliary Lane Research Articles

Potential Updates to 2004 Green Book's Acceleration Lengths for Entrance Terminals

AASHTO's Policy on Geometric Design of Highways and Streets (Green Book) notes that drivers entering a highway from a turning roadway accelerate until the desired highway speed is reached. Because the change in speed is usually substantial, provision is made for acceleration to be accomplished on an auxiliary lane, called an acceleration lane, to minimize interference with through traffic and to reduce crash potential. The 2004 Green Book contains acceleration lane lengths. The procedure identified to reproduce these values assumed running speed for the limited-access highway and the ramp along with acceleration rates from 1930s studies. Potential acceleration length values were then calculated by (a) updating the assumptions within the identified procedure and (b) using spreadsheets that can generate second-to-second acceleration. Findings from recent studies were compared with the existing Green Book values and the calculated suggested acceleration lengths. The suggested lengths determined in this paper, which are based on more realistic speed assumptions, more current acceleration lengths, and findings from recent research, are longer than the values in the Green Book. The paper recommends that additional research be done on acceleration lengths to determine whether the Green Book values should be increased.

DERIVATION OF CONTINUUM TRAFFIC MODEL FOR WEAVING SECTIONS ON FREEWAYS

This paper presents a new continuum model describing the dynamics of multiclass traffic flow on multilane freeways including weaving sections. In this paper, we consider a specific freeway weaving type, which is formed when an on ramp is near to an off ramp and these two ramps are joined by an auxiliary lane. Traffic interactions in this weaving zone are very complex due to the involvement of weaving flows and non-weaving flows in the so-called mandatory lane-changing process. To handle this complexity, it is essential to have a good understanding of the (microscopic) driving behavior within the weaving zones. These behaviors are modeled based on a gap-acceptance model. The methodology to obtain a weaving continuum traffic model is thus twofold. On the one hand, we develop a (macroscopic) model to determine the mandatory lane-changing probability based on a renewal process. On the other hand, we implement the lane-changing model into a current gas-kinetic traffic flow model for heterogeneous traffic flow on multilane roadways. From this, corresponding macroscopic model is obtained based on the method of moments.

A Study on Bottleneck Capacity on Divided Two-lane Expressways

The paper deals with the bottleneck phenomenon and capacity analysis of the divided two-lane expressways in Japan, which has not been studied so far, although those of the multi-lane expresswayshave been done sufficiently. It is found from the observed data that (1) the bottleneck phenomenon of capacity reduction from the maximum flow rate just before congestion to that discharged from the congestion queue behind bottleneck occurs on the divided two-lane expressways as similar to that of multi-lane expressways;(2) the capacity reduction for the divided two-lane expressways is largerthan that for the multiple-lane expressways;(3) the bottleneck capacity of the divided two-lane expressways is less than the average capacity per lane of the multi-lane expressways. In additions, (4) the relationship between bottleneck capacity and highway geometric characteristics is studied. Finally it suggests a mechanism of improvement of bottleneck capacity by adding an auxiliary lane upstream of the bottleneck for the divided two-lane expressways.

Observed Dynamic Traffic Features on Freeway Section with Merges and Diverges

Traffic data measured on a four-lane freeway and its on-ramps (with metered flows) and off-ramps were studied for 4 days during the peak hour. Certain features were reproducible from day to day. A bottleneck arose on all 4 days because of traffic merging and diverging at a freeway location. During these times queuing arose in a freeway section with an auxiliary lane some distance downstream of the merge. Corresponding flow reductions were also measured at a downstream off-ramp. Queuing was apparently caused by drivers who had just entered the shoulder lane from the on-ramp and had slowed down to merge. The slowing of vehicles spread to both lanes and propagated upstream. The average discharge flows that accompanied the onset of queuing were 4% lower than flows measured just before the queue's formation. Upon bottleneck activation, flow reductions occurred sequentially in time and space marking the passage of the backward-moving shock. Mean shock velocities ranged from 20 to 24 mph (32 to 40 km/h). The analysis used transformed curves of cumulative vehicle counts and cumulative occupancy to obtain the measurement resolution necessary for studying important traffic features. The site is located in Minneapolis, Minnesota, and data were collected before a ramp meter shutdown experiment conducted in 2000.

Effect on Utilization of Auxiliary through Lanes of Downstream Right-Turn Volume

To increase the capacity of through traffic at signalized intersections, additional lanes with limited lengths—called auxiliary lanes—are usually added to the roadway at the intersection. Because of their limited length, as well as other factors, these lanes are not fully utilized as other continuous through lanes. The objectives of this research were (1) to identify the level of utilization of auxiliary through lanes added at intersections of four-lane roadways with right-turn driveways and/or side streets present within the downstream length of the auxiliary lane; and (2) to study the effect of auxiliary lane length, right-turn volume existing downstream into driveways/side streets, and through/right-turn lane group delay on the level of their utilization. Lane-use and right-turning volume data collected during 866 saturated cycles at eight signalized intersections with different auxiliary lane lengths and right-turn volumes were used to accomplish the research objectives. Auxiliary lane length and right-turn volume downstream were found to significantly contribute to the use of auxiliary lanes at the 0.01 level of significance. Lane-use of nearly 0.4–1.9 straight-through vehicles per cycle, depending on lane length and downstream right-turn volume, was observed at the study locations. Though not directly comparable to this study, previous studies had indicated lane-use values within this range. Longer auxiliary lanes and lower downstream right-turn volumes encouraged the use of auxiliary lanes by straight-through vehicles. The range of lane utilization adjustment factors (\If\DLU\N-factors) calculated from field data was 0.72–0.80, which is lower than the 1997 U.S. Highway Capacity Manual default value of 0.91 for a three-lane through/right-turn group.

A methodology for estimating capacity at ramp weaves based on gap acceptance and linear optimization

Freeway weaving areas are those formed when the paths of two or more traffic streams cross, i.e., when a merge area is closely followed by a diverge area, and the two are joined by an auxiliary lane. Previous research on weaving areas has mostly focused on developing methods for estimating speeds for the weaving and non-weaving traffic streams, and on determining freeway weaving level of service; there are no methods available for estimating freeway weaving capacity. The recently published Highway Capacity Manual (HCM, 2000) presents capacity values for weaving areas that are solely based on a density of 27 pc/km/ln. The objective of this study is to develop a method for estimating capacity of ramp weaves based on gap acceptance and linear optimization. The methodology provides estimates of the capacity of ramp weaves, as a function of the capacity of the equivalent basic freeway segment lane, and for given proportions of origin–destination demands within the weave. The paper presents the methodology along with results of the capacity estimation for the range of possible weaving and non-weaving flows. It also presents the results of sensitivity analyses conducted to assess the impact of the assumptions used in the methodology. Finally, the results obtained by this method are compared to the HCM (2000) capacity values.

Practical Research on Auxiliary Lane Controlling Traffic Lane Distribution

This paper verified the ways to control traffic lane distribution at several auxiliary lane sections in uninterrupted flow to enlarge the capacity at bottlenecks, in macroscopic manners and quantitatively, by using traffic detector data. The effect of auxiliary lane placement to reduce congestion was evaluated in termsof its length or the distance from its end to the bottleneck downstream. The temporary auxiliary lane emerged during the intensive maintenance works in Tomei Expressway. Thus, both volume and velocity lane distributions were analyzed to evaluate its effect to mitigate congestion.

Utilization of Auxiliary Through Lanes at Intersections of Four-Lane, Two-Way Roadways

To increase the capacity of through traffic at signalized intersections, additional lanes with limited length—called auxiliary lanes—are added to the roadway at the intersection. Because of their limited length, as well as other factors, these lanes are not as fully utilized as other continuous through lanes. Research was undertaken with two objectives: ( a) to observe and identify the level of use of auxiliary through lanes added at intersections of four-lane, two-way roadways; and ( b) to study the effects of auxiliary lane length, right-turn volume, and through/right-turn lane group delay on the level of their use. Lane-use data collected during 1,050 saturated cycles at eight signalized intersections with different auxiliary lane lengths were used to accomplish research objectives. All factors investigated—auxiliary lane length, right-turn volume, and stopped-delay—were found to contribute significantly to the use of auxiliary lanes at 0.01 level. The level of each factor’s contribution, however, was dependent on the level of the other two. Lane use of nearly one to seven straight-through vehicles per cycle, depending on levels of factors investigated, was observed at the study locations. Longer auxiliary lanes, lower right-turn volumes, and excessive approach delays encouraged the use of auxiliary lanes by straight-through vehicles. The range of lane utilization adjustment factors ( fLU-factors) calculated from field data was 0.73 to 0.82, which is lower than the 1997 Highway Capacity Manual default value of 0.91 for a three-lane through/right-turn group.

Maximum Possible Weaving Volume for Effective Operations of Ramp-Weave Areas: Online Estimation

Understanding the behavior of weaving flows and estimating the effects of time-variant traffic conditions on the capacity of weaving areas are of critical importance in developing effective operational strategies for freeway management. An online procedure for estimation of the maximum possible weaving volume over time for a given ramp-weave area is presented. The method is based on the findings from weaving behavior analysis, which indicates that under moderate to heavy flow conditions the diverging vehicles, that is, the freeway-to-ramp vehicles, tend to move to the auxiliary lane on entering the weaving area, whereas the ramp-to-freeway vehicles travel a short portion of the auxiliary lane before they merge to the main line. Therefore, the beginning portion of an auxiliary lane in a ramp-weave area is shared by both merging and diverging vehicles, and the length of such a shared area, called the effective weaving zone, varies depending on the length of a given weave section and the amount of weaving flow. Such join-then-split weaving behavior implies that the maximum value of the weaving volume in a ramp-weave area with one auxiliary lane is limited by the maximum through capacity of one lane. The proposed method reflects the weaving behavior described above and determines the maximum possible weaving volume for a given time interval as a function of exit and merge capacities, which are dependent on the traffic conditions downstream of weaving areas. Preliminary test results with 5 min of data from a ramp-weave site indicate that the maximum possible weaving volume is estimated with reasonable accuracy.

Partnering on a Design-Build Project: Making the Three-Way Love Affair Work

The partnering process used by the Arizona Department of Transportation in the execution of an $89 million design-build reconstruction of an urban freeway through a congested section of Phoenix is described. The project is changing 6 lanes into 10 lanes by adding a high-occupancy vehicle lane, along with auxiliary lanes, between the entrance and exit ramps over a 13-km (8-mi) stretch of freeway. It involves the demolition and replacement of two bridges that carry major arterial roads over the freeway by using single-point urban interchanges along with several kilometers (miles) of sound walls, new freeway lighting, and an automated freeway management system. Design-build by its nature lends itself to the partnering concept. The partnering concept ideas of increased communication, alignment of goals, and development of a dispute resolution system fit perfectly with design-build’s overarching theme of single-point responsibility for the owner. Increased pressure because of schedule compression typical of most design-build projects makes partnering a vital necessity. Several innovative partnering ideas used on the design-build project to overcome the problems inherent in a complex, high-profile, fast-paced construction project are described.

Freeway Facility Methodology in Highway Capacity Manual 2000

The next edition of the Highway Capacity Manual (HCM 2000) will contain for the first time an operational analysis procedure for directional freeway facilities up to 20 to 25 km long. At the simplest level, this procedure integrates the proposed HCM 2000 methods for the analysis of basic, ramp, and weaving segments to enable the analysis of an entire facility. But the proposed facility methodology goes much further. It allows the user to analyze multiple, contiguous time intervals with timevarying demands and capacities. It can handle both undersaturated and oversaturated traffic conditions (with some limitations). In the latter case, both the spatial and time extent of congestion are estimated. Finally, the method permits the investigation of the effect of many traditional and intelligent transportation system–based freeway improvement strategies such as full or auxiliary lane additions, ramp metering, incident management, and a limited set of high-occupancy-vehicle designs on facility performance. Described here is the conceptual model for and computational steps of the methodology, with emphasis on the components for analysis of oversaturated conditions. The scope and limitations of the methods are also highlighted. Reference is given to a companion paper that describes how the results of the method were validated in the field and how they compared with those obtained from widely used freeway simulation models.

Utilization of Double Left-Turn Lanes with Downstream Lane Reductions

The capacity of left-turn movements at signalized intersections is sometimes increased by using two exclusive double left-turn lanes at the intersection followed by a reduction to a single lane after the turn is made. A typical application is the use of double left-turn lanes onto a freeway entrance ramp, where the right lane is dropped in advance of the merge area. The effectiveness of the “added” outside left-turn lane and downstream auxiliary lane depends on the amount of traffic using that lane. While equal distribution of traffic between a continuous and an auxiliary through lane would result in the greatest total capacity of this lane pair, traffic and geometric factors are usually such that this does not occur. The 1994 Highway Capacity Manual does not address intersection configurations with downstream lane reductions. This paper describes the development of a mathematical model, based on field data, which does predict auxiliary lane use for this intersection configuration.

Utilization of Auxiliary Through Lanes at Signalized Intersections with Downstream Lane Reductions

The capacity of multiple through lanes at signalized intersections depends on the distribution of traffic within these lanes, with equal lane distribution corresponding to maximum capacity. However, traffic characteristics, land use, and geometric factors usually prohibit this from occurring. Although the 1994 update of the Highway Capacity Manual considers the case of continuous through lanes at signalized intersections, the default values provided do not address situations in which lane reduction takes place downstream of the intersection. Lane distribution data obtained in the field can remedy the situation but for existing conditions only. This research employed the concept of captive and choice lane users in modeling lane use for intersection configurations with a single continuous through lane and an “auxiliary” through lane, which is continuous upstream of the intersection but is dropped downstream of it. Stepwise multiple regression was performed on data collected at sites in Tennessee to ascertain those factors significantly affecting auxiliary lane use. These factors were found to be ( a) right turns off the facility at the intersection, ( b) total left turns off the facility downstream of the intersection, ( c) right turns onto the facility in the first 122 m (400 ft) upstream of the intersection, ( d) right turns off the facility in the last 152 m (500 ft) of the auxiliary lane, ( e) downstream auxiliary lane length, and ( f) the existence of left-turn bays or two-way continuous left-turn lanes downstream of the intersection. For the configuration studied, lane distribution data often differed considerably from the default values given in the Highway Capacity Manual.

Cross-section elements to accommodate passing lanes and vehicle storage during avalanche control for the Trans-Canada Highway in Rogers Pass

The two basic problems facing the Trans-Canada Highway through Rogers Pass, namely the winter problem of adequate vehicle storage areas during avalanche control and the summer problem of maintaining an acceptable level of service by ensuring adequate passing opportunities, may be linked through a common denominator — passing lanes. This paper presents the findings of a recently completed study of the Trans-Canada Highway through Mount Revelstoke and Glacier National Park. The study method is based on the fact that traffic flow characteristics, highway conditions, and operational requirements vary considerably from season to season. Thus, the analysis was broken into two parts, namely a separate analysis of winter and summer conditions. A traffic simulation model was used to determine those locations that could serve as potential sites for passing lanes during summer. The traffic simulation model developed for the passing lane analysis utilized files that describe road geometry, summer traffic flow and composition, and vehicle performance. A separate simulation model was developed to determine vehicle storage requirements of ponding areas during avalanche control. The model developed for winter traffic characteristics also provided the basis for a flexible traffic management strategy. Cross-section elements were developed to accommodate an auxiliary lane to be used as a passing lane in summer and as a parking lane to increase the capacity of ponding areas during avalanche control in winter. Design requirements called for triple stacking of vehicles in ponding areas during avalanche control. To accommodate design year traffic volumes at an acceptable level of service during the summer months, 12 passing lanes totalling 23 km in length were recommended. Six ponding areas, totalling 4 km in length, are coincident with four of the passing lane locations. The paper concludes with a general discussion on the benefits of linking traffic simulation with geometric design to demonstrate how the design life of rural two-lane highways can be extended by the use of low-cost operational improvements such as passing lanes. Key words: highway planning and engineering, two-lane highways traffic management during avalanche stabilization, passing lanes, cross-section elements, Trans-Canada Highway.