Student Number: 201309BLX0017

SOUTH CHINA UNIVERSITY OF TECHNOLOGY

Report
HIGHWAY DESIGN
By
PHD STUDENT
HADI HUSSEIN ALI ABDULLAH

College: CIVIL ENGINEERING

Major: TUNNELS ENGINEERING
Research: Report
Student Number: 201309BLX0017
Prof : 符锌砂 FÚ XĪN SHĀ

April 2014
 HIGHWAY DESIGN REPORT

Introduction:

Foreword this report documents a safety review of a significant highway project prior to
its opening. The mandate of the Highway Safety Review Committee was to carry out the
review from a safety Perspective, and we did not attempt to examine such broader
considerations as levels of service and Economics. Nor did we examine questions of
responsibility, authority, legal obligation, or related matters, since these were neither
relevant to a safety evaluation of the road, nor to the technical mandate of our
committee.

1. Purpose

The purpose of this report is to:

1- Describe in short the general design and safety performance of modern roundabouts,

2- Present a proposal for design principles for modern roundabouts in Turkey.

The report is focused on safety issues. In addition, there are a number of issues that
must be included in future guidelines, for example, capacity and delay calculations and
design of cross fall and drainage.

The objective is to incorporate modern roundabouts as a standard intersection type in

new comprehensive Turkish design guidelines. To work out new design guidelines is
however a long procedure. It is suggested, therefore, that the proposed design
principles are adapted to Turkish conditions and used for the introduction of modern
roundabouts

2 Terminology

To distinguish from “traffic circles” in general and the “rotaries” already used, it is
suggested that the term modern roundabout is used for roundabouts designed and
controlled according to the principles in this report.

The main design elements of a modern roundabout are shown in the figure below.
 Fully directional four-level interchange

Operation

Partially directional interchange with inner loop ramps

Right turn
Approach on the right, stay on the right in the roundabout and leave on the right.
Display your right turn indicator on the approach and through the roundabout

Straight ahead

Design speed is appropriate. Selection of ramp design depends upon the type of
intersecting roads and

The site controls. For outer loops and direct ramps from crossing roads, the standard
values of design

Speed if the right lane is blocked due to dense traffic or road obstruction, then approach
and continue through the roundabout in the left lane (broken line).

In both cases, display your right turn indicator just past the exit prior to the one you
plan to take. (No signal is required on the approach to the roundabout).

Advantages and disadvantages

Compared to priority controlled and signalized intersections, some of the advantages

and disadvantages with roundabouts are pointed out in the tables

Advantages Priority controlled Signalized intersections

intersections

Safety Reduced number of conflict points compared

to uncontrolled intersection.

Lower speeds reduce frequency and severity

of both vehicle-vehicle and pedestrian-
 vehicle accidents.

Capacity Traffic yields rather Roundabouts should normally

than stops, often give higher capacity/lane due to
resulting in the the omission of lost time (red
acceptance of smaller and yellow) at signalized
time gaps. intersections.

Delay Overall delay will probably be less than for

an equivalent volume signalized intersection.

Cost Accident costs are low due to the low

number of accidents and severity.

Less right-of-way is generally Maintenance costs are generally lower for

required. roundabouts.

Pedestrians/ bicyclists Low speeds reduce frequency and severity of

pedestrian-vehicle accidents.

Accident and injury rates

Based on the Swedish study, prediction models have been developed for accident and
injury rates. The models take into account the speed limit, number of legs and number
of lanes in the roundabout. The models give the following rates per million incoming
vehicles.

4- leg 3- leg
Local General 1-lane 2-lane 1-lane 2-lane
speed limit speed limit
50 km/h Same as 0.113 0.133 0.109 0.127
local
70 km/h -"- 0.208 0.244 0.200 0.234
50 km/h Higher 0.159 0.186 0.152 0.178
than local
70 km/h -"- 0.292 0.341 0.280 0.328

Proposed design principles

2.1 The use of modern roundabouts

According to recent studies roundabouts are safer than signalized intersections.

Consequently, for safety reasons roundabouts should be selected if priority intersections
cannot be used.

Roundabouts generally can be accepted on all roads and in all locations where control
intersections are accepted. The following table shows an example on a policy for the
acceptance of roundabouts.

Road class
Location State road type I State road type II Provincial road type
I and II
Rural - Accepted Accepted
Suburban Accepted Accepted Accepted
Urban Accepted Accepted Accepted
Design vehicles
A roundabout should be designed to give good accessibility for the type of vehicles
which normally use the roundabout and to be able to pass with special vehicles. Some
Swedish design vehicles are shown in the pictures below. (Note that the semi-trailer is
rather similar to the semi-trailer C50 in the Turkish guidelines.

Example of normal design vehicles

Passenger car, total length 5,0 m

Semi-trailer, total length

16,0 m

Special trailer, total length 19,0 m

Basic design elements

The basic design elements, which determine the size of a roundabout, are:

 the radius of the central island,

 the number of lanes,
 the width of the circulating roadway.

Radius of the central island

The central island radius for a normal roundabout should generally be between 10 and
25 meters. For two lanes roundabouts it should be at least 15 meters.

In some European countries small roundabouts with a central island radius between 2
and 10 meters and mini-roundabouts with central islands less than 2 meters are used in
urban areas. These types are generally not suitable on rural roads and city passings
since they cannot be used by all long vehicles. Consequently, these types are not
suggested to be used on KGM roads.

There is no simple relation between the central island radius and the (possible)
capacity. The capacity is also depending on other design parameters, such as the width
of the circulating roadway and the entry and exit angles and widths. This can be
understood by the figure below which shows present traffic volumes (not capacity) for
about 180 existing Swedish roundabouts, However, roundabout with bigger central
islands have higher capacity because both the time gaps between vehicles and the
lengths of the weaving sections are longer.

Entering traffic, v/d

Central island radius, m

Existing traffic volumes in Swedish roundabouts with different central island radii.

Detailed design

From a safety point of view, the most important design elements are:

 entries,
 exits,
 pedestrian and bicycle crossings.

Normal design of these elements is shown in the examples in chapter 3.

2.3.1 Entries and exits

The entry should be designed to encourage the drivers to lower their speeds. This is
done by choosing a suitable design of the entry curve and in some cases by a flare of
the entry.

The exit should be designed to give smooth traffic flow.

Entry flare

The purpose of a flare of the entry is to reduce the speed. Consequently, it should be
used where the speed on the connecting road is high before the roundabout, primarily
in rural areas with a speed limit of 70 km/h or more.

The size of the flare is depending on the alignment of the entry and should normally be
about one lane wide (3.75 m).

Entry curve

The entry curve should be designed like an entry curve in an ordinary priority
intersection. The main radius should normally be around 15 meters.

Exit curve
The main radius in the exit curve should be between 100 and 200 meters for a normal
symmetric roundabout. If there is a pedestrian crossing, the radius should be smaller.

Combination of exit and entry curves

The alignment of the connecting roads can make it necessary to adjust the exit and
entry curve radii. If larger radii than normal are used, it must be checked that there are
no possible driving paths for passenger cars with a radius more than 100 meters
according to the figure on page 8.

If possible, between the entry and the following exit, there should be no part of the
outer roundabout circle. For roundabouts with big central island radii or long distances
between entry and exit, this can be difficult to avoid. If possible, the alignment of the
connecting roads should be adjusted.

One lane urban roundabout with pedestrian crossings

Central island radius: 15 m (included 1 m driving area for heavy vehicles)

Outer radius: 21 m

Entry radius: 15 m

References

1- Exit radius: 50 m107. Hamilton Associates. Discussion Document (13-page

document).

2. Inner Loop Ramp Radius Study, R.W. Oddson, March 1990.

3 . Central Region Collision Analysis Report 1991-1993.

4. Highway 401 Accident Analysis, London to Woodstock, August 1995.

5. Traffic Operations Study: Highway 403, QEW to Highway 6, October 1993.

6. Highway 401 Windsor to Cambridge, July 1993.

7. Median Barrier Planning Report: Highway 401, June 1993.

8. Highway 401 Woodstock to Kitchener Accident Analysis, April 1993.

9. Box Beam Barrier Performance: An Operational Review, March 1993