Lec 07 Highway Engineering - Speed Change Lanes

Lecture 07 79

Highway Eng. Speed Change Lanes 14 15

Dr. Firas Asad

In this lecture;

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A- Definition and Purposes

B- Right-Turn Lane.

C- Left-Turn Lane.

D- Iraqi HDM Specifications

Speed Change Lanes (Turn Lanes)

The information listed in this lecture is mainly taken from the Policy on Geometric Design of Highways and Streets (AASHTO, 2011), Iraqi Highway Design Manual (SORB, 2005) and TxDot Highway Design Manuals (TxDot HDM, 2014).

A- Definition and purposes

Drivers leaving a highway at an intersection are usually required to reduce speed

before turning. Similarly, drivers entering a highway from a turning roadway

accelerate until the desired open-road speed is reached.

Hence, a Speed-Change Lane is an auxiliary lane, including tapered areas, primarily

for the acceleration or deceleration of vehicles entering or leaving the through-

traffic lanes at intersections or interchanges. The terms speed-change lane,

deceleration lane, or acceleration lane, usually refer to the added pavement

joining the travelled way of the highway/street with that of the turning roadway.

A speed-change lane should be of sufficient width and length to enable a driver to

manoeuvre a vehicle into it properly, and once into it, to make the necessary change

between the speed of operation on the highway/street and the lower speed on the

turning roadway.

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Dr. Firas Asad

This lecture mainly deals with the speed change lanes at Intersections

Auxiliary lanes may also be added to increase capacity and improve safety at an

intersection. In many cases, an auxiliary lane may be desirable after completing a

right-turn movement to provide for acceleration, manoeuvring, and weaving.

; especially the

design elements of auxiliary lanes of left-turning movements at median openings

and auxiliary lanes of right-turning movements at intersections.

Auxiliary lanes should be at least 3.0 m wide

The

and desirably should equal that of the

through lanes.

length

of the auxiliary lanes for turning vehicles consists of three components:

(1) entering taper, (2) deceleration length, and (3) storage length. Desirably, the

total length of the auxiliary lane should be the sum of the length for these three

components. Common practice, however, is to accept a moderate amount of

deceleration within the through lanes and to consider the taper length as a part of

the deceleration within the through lanes. Each component of the auxiliary length is

discussed in the following section.

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Figure: Design of left-turn lanes for roads without median.

B- Left-Turn Lane Design.

1) Entering taper

To develop the width needed for auxiliary lanes, a transition must be effected. This

transition, or taper

An approach taper provides space for a left-turn lane by moving traffic laterally to

the right on a street/highway

, allows a driver to recognize that an exclusive lane is being

developed and also allows some deceleration to occur prior to entering the storage

lane itself. It is a common practice to consider the taper length as a part of the

deceleration lane. There are two distinct tapers commonly used in the left-turn lane

design: (1) approach taper length and (2) bay taper length.

without a physical median

The bay taper length is a reversing curve along the left edge of the travelled way

that directs traffic into the left-turn lane.

.

The first figure below shows the geometrical design of a left-turn lane when there is

no physical median, while the second figure is when median is exist.

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Dr. Firas Asad

Figure: Design of left-turn lanes when median is available.

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Dr. Firas Asad

2) Deceleration lane length

For median left-turn lanes, a minimum median width of 4.8 m (3.6 m lane width plus

a 1.2 m separator) is recommended to accommodate a single left-turn lane.

According to the online Road Design Manual (Texas department of transportation)

(http://onlinemanuals.txdot.gov/txdotmanuals/rdw/urban_streets.htm#CIHJCABA)

the recommended lengths are shown in the following table.

3) Storage lane length

The auxiliary lane should be sufficiently long to store the number of vehicles likely to

accumulate during a critical period. The storage length should be sufficient to avoid

the possibility of left-turning vehicles stopping in the through lanes waiting for a

signal change or for a gap in the opposing traffic flow.

At unsignalized intersections, the storage length may be based on the number of

turning vehicles likely to arrive in an average 2-minute period within the peak hour.

Space for at least two passenger cars should be provided; with over 10% trucks, at

least one car and one truck.

The required storage can be obtained using traffic models such as those in the HCM

software (HCS) or alternatively using the following:

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L = (V/N) (2) (S)

where:

L = storage length in meters.

V = left-turn volume per hour, vph.

N = number of cycles if signalised or 30 if unsignalised.

2 = a factor that provides for storage of all left-turning vehicles on most cycles.

S = queue storage length, in meters, per vehicle. Typically 7.5m for automobiles.

C- Right-Turn Lane Design.

Figure below shows a right-turn deceleration lane. Basically, the required length for

a right-turn lane is calculated in the same manner as described for left-turn lanes.

The length and width of taper is the same as in the left-turn lane. The length of a

single right-turn deceleration lane is the same as that for a single left-turn lane (see

Table in p.82). However, the minimum queue storage is 9.0 m for right-turn lanes.

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Acceleration Lanes

Acceleration lanes for right-turning and/or left-turning vehicles may be desirable on

multi-lane rural highways. Generally, design principles of the deceleration lanes are

applicable for acceleration lanes (see Figure below). Trucks need longer speed

change lanes for acceleration.

Acceleration lanes are not always desirable at stop-controlled intersections where

entering drivers can wait for an opportunity to merge without disrupting through

traffic. Acceleration lanes are advantageous on roads without stop control and on all

high-volume roads even with stop control where openings between vehicles in the

peak-hour traffic streams are infrequent and short.

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D- Iraqi Highway Design Manual (SORB): Speed Change Lanes

Documents

Lec 07 Highway Engineering - Speed Change Lanes