Cover Image

Not for Sale

View/Hide Left Panel
Click for next page ( 22

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 21
21 2001 Green Book (3). Addressing the unique nature of inter- operations. The stopped vehicle must first have adequate sections on divided highways, the report states that these sight distance to depart from a stopped position and cross intersections may have substantial sight-distance concerns traffic approaching from the left. The crossing vehicle may for left-turning vehicles. Despite the provision of stopping then stop in the median prior to performing the second oper- sight distance (SSD) along each roadway, sight obstructions ation. The second move requires the necessary sight distance in the median could limit left-turn sight distance. Further- for vehicles to depart from the median, to turn left into the more, opposing left-turn vehicles on divided highways may crossroad, and to accelerate without being overtaken by vehi- be aligned in such a way that they become sight obstructions cles approaching from the right. to one another, blocking the view of oncoming traffic on the The handbook also presents recommended sight distance major road. The sight restrictions created by opposing left- values for U-turns at unsignalized median openings--these turn vehicles can be minimized by the use of parallel and values are provided here in Table 8. tapered offset left-turn lanes. Finally, the Florida Median Handbook (8) discusses sight NCHRP Report 375 (7) recognizes that ISD at divided distance issues related to opposing left-turn vehicles and sug- highway intersections is complicated by the presence of the gests that vehicles turning left from opposing left-turn lanes median on the major road, which may increase the ISD restrict each other's sight distance unless the lanes are suffi- requirements at some intersections or may contain sight ciently offset. A positive offset of 0.6 m (2 ft) is recommended obstructions that reduce the ISD. The Green Book (3) con- when the opposing left-turn vehicle is a passenger car and 1.2 siders ISD to be adequate when drivers at, or approaching, an m (4 ft) when the opposing left-turn vehicle is a truck. intersection have an unobstructed view of the entire inter- section and of sufficient lengths of the intersecting highways to permit them to anticipate and avoid potential collisions. Adequate ISD requires unobstructed sight distance along INDIRECT LEFT-TURN MANEUVERS both approaches of both intersecting roadways, as well as across the clear sight triangles. Adequate clear sight triangles Indirect left-turn maneuvers include the use of jughandle are required both for drivers approaching an intersection roadways before a crossroad, loop roadways beyond a cross- where they are not required to stop and for drivers who are road, and directional median openings beyond a crossroad. stopped at an intersection waiting to proceed safely to cross Indirect left-turn treatments enable drivers to make left turns a major roadway or to turn left or right onto a major roadway. efficiently on divided highways, including highways with rel- ISD requirements for crossing and turning maneuvers at atively narrow medians. The Michigan and New Jersey DOTs divided highway intersections are generally increased with have used indirect left-turn treatments extensively; other state median width until the median becomes wide enough to store highway agencies have used them occasionally (7). Increas- a vehicle. If the median is wide enough to store a vehicle, then ingly, Florida is limiting unsignalized median openings to left the intersection operates as two separate intersections, because turns from the arterial roadway; hence, drivers wishing to turn drivers can cross the near roadway and stop in the median, if left from a driveway must turn right and then make a U-turn or necessary, before crossing or turning into the far roadway. In use some other alternative route. Design policies concerning this case, the sight distance requirements of the intersections with the two roadways of the divided highway can be deter- TABLE 8 Sight distance for U-turns at unsignalized mined separately. median openings (8) NCHRP Synthesis of Practice 281 (54) discusses alterna- Speed (mph) Sight distance (ft) tive improvement techniques that can be implemented to mit- 35 520 igate the problems encountered by larger vehicles at divided 40 640 45 830 highway intersections with narrow medians. When sight dis- 50 1,040 tance for left-turn vehicles is limited by opposing through 55 1,250 vehicles, this report recommends the following mitigation 60 1,540 Speed (km/h) Sight Distance (m) techniques at unsignalized intersections: 60 160 70 200 Offset opposing left-turn lanes by moving them laterally 80 260 90 380 within the median. 100 470 Prohibit left turns from the major road. Assumptions: Close the median opening. Design vehicle = passenger vehicle Reaction time = 2.0 sec Require indirect left-turn movements. Extra time needed in the U-turn maneuver U-turn vehicle begins acceleration from 0 mph only at the end The Florida Median Handbook (8) acknowledges that of the U-turn movement Values are based on speed/distance/acceleration figures crossing and turning maneuvers onto a divided highway from from the 1990 AASHTO Green Book a minor road or driveway can be performed as two separate 50-ft clearance factor

OCR for page 21
22 indirect left-turn treatments are addressed in the AASHTO Figure 10 illustrates a design that provides for indirect left Green Book (3). turns to be made from the right, via separate turning road- Indirect left turns and U-turns are discussed on pages 709 ways connected to a crossroad. Such arrangements eliminate through 716 of the Green Book (3). Several design alterna- left turns from the through lanes and provide storage for left- tives are presented. Figure 8 presents a jughandle-type ramp turning vehicles not available on the highway itself. The left- or diagonal roadway that intersects a secondary crossing road- turning vehicles are able to cross the main highway with lit- way. The driver exits via the jughandle-type ramp and makes tle extra travel time. a left turn onto the crossroad. For a U-turn maneuver, the Figure 11 presents an indirect left turn for two arterials driver makes an additional left turn onto the divided highway. where left turns are heavy on both roads. Because lack of Figure 9 shows an at-grade loop that may be considered storage for left turns from the minor road would cause con- when the jughandle-type ramp would require costly right-of- gestion, left turns from the minor road are prohibited. Left- way. Other factors favoring the at-grade loop include verti- turning traffic turns right onto the divided road and then cal alignment and grading costs. makes a U-turn at a one-way crossover located in the median Figure 8. Jughandle-type ramp with crossroad (3). Figure 9. At-grade loop (surface loop) with crossroad (3).

OCR for page 21
23 Figure 11. Indirect left turn through a crossover (3). incorporate an entirely different route from origin to destina- tion, so that the larger vehicle can make a right turn into the driveway at its destination. Where the median width of a divided highway at a median opening is narrow, no left-turn lane is provided, and the opposing traffic flow is high, drivers of larger vehicles that want to make a left turn may recognize that the median opening does not have sufficient size to accommodate their vehicle and that stopping in a through traffic lane to wait for a gap in opposing traffic leaves them potentially exposed to rear-end collisions. In this situation, Figure 10. Special indirect left-turn designs for traffic drivers of larger vehicles may proceed to the next major leaving highway with narrow median (3). intersection to complete a U-turn maneuver or may use an indirect route to their destination, just as they would if no median opening were provided. There are no generally applic- able estimates concerning how much delay to larger vehicles of the divided road. Auxiliary lanes are highly desirable on may result from such indirect routings. each side of the median between the crossovers for storage NCHRP Report 420 (4) reports an estimated 20-percent of turning vehicles. reduction in accident rate by replacing direct left turns from In a series of ITE articles (58, 59), Hummer described driveways with right-turn/U-turn treatments. Table 9 sum- seven unconventional left-turn design alternatives for urban marizes the differences in accident rate at three unsignalized and suburban arterials. The alternatives share two major prin- locations where direct left turns were replaced by indirect ciples: (1) reduce delay to through vehicles and (2) reduce and left turns. separate the conflict points at intersections. Hummer and Reid Levinson et al. (31) present the safety and operational ben- recently reviewed five of the seven alternatives--the median efits of prohibiting left turns at signalized intersections along U-turn, bowtie, superstreet, jughandle, and continuous flow divided arterials in Michigan and installing directional U-turn intersection--and summarized new information about each crossovers downstream. Key features of the indirect left-turn (60). After presenting the advantages and disadvantages of treatments include the following: each alternative, the authors suggest when analysts should consider each alternative during feasibility studies and func- Two-phase signal operation at the major intersection tional designs. where all left turns are prohibited; NCHRP Synthesis 281 (54) presents a discussion of indirect Directional U-turn crossovers for left turns located about left-turns by larger vehicles. The report states that although the 200 m (660 ft) on each side of the signalized intersection; denial of left-turn access by a raised median is likely to Right-turn lanes on the major and minor roads; increase U-turn demand at nearby median openings, it is also Left-turn lanes in the median of the major road for U-turn likely that some larger vehicles will use indirect routes that crossovers; do not involve a U-turn maneuver to reach their destination. Coordination of signals in each direction of travel along Such routes may involve going around the block or may the major road to ensure progressions; and