CONVENTIONAL ROUNDABOUTS: THEIR EFFECTIVENESS AS

ROAD INTERSECTION CONTROL

Y.A. Abdul Kareem Civil Engineering Department,

University of llorin, llorin. NIGERIA.

ABSTRACT

The at- grade intersections form over 90% of the road network systems in the nation's highway system. Of these intersections, the conventional roundabout is one of the most effective, efficient and safe intersections found in urban areas the aim of this paper is to study three of such conventional roundabouts in llorin City of Kwara State. Their capacities, delay to motorists and average speed of vehicles in the periphery of the roundabouts were measured in order to find how efficient and effective they are. The paper finally suggests ways and means of redesigning them for better and safer performance.

Keywords: Roundabout, Effectiveness, Management, Control.

INTRODUCTION

At a multi-arm road junction where traffic flows are low. Traffic control may be achieved by the priority type of control the form of priority control in this country is that minor road traffic gives way to major road traffic. When traffic flow increases, delays are experienced at such a priority type of intersection and at high flows, other forms of traffic control devices become necessary in this country, most multi-arm road intersections with high traffic volume are controlled by the rotary type of intersection popularly known as the roundabout.

TYPES OF ROUNDABOUTS

There are generally five types of roundabouts and these are:

a) Conventional Roundabout which is composed of a circular or asymmetrically large central island usually more than 25 meter diameter around which there is a one-way carriageway with weaving sections. The carriageway may or may not have flared approaches

b) Small Roundabout: With a one-way circulatory carriage-way round a small central island less than 25 meter diameter with flared approaches

c) (c) Mini Roundabout: This is a roundabout with one way carriage way around a flush or slightly raised circular island less than four meters diameter with or without flared approaches

(d) Double Roundabout: An individual junction with two small or mini roundabouts either contiguous or connected by a short link road.

(e) Multi Roundabout: An individual junction with three or more small or mini roundabouts either contiguous or inter connected by short link roads.

GENERAL DESIGN FEATURES

The primary design areas are to provide the following features:

(a) Adequate entry width

(b) Adequate circulation space compatible with the entry width. (c) Central island of diameter sufficient only to give drivers guidance on the maneuvers expected. (d) Deflection of traffic to the right of entry to promote gyratory movement. (e) Adequate deflection of crossing movements to ensure low traffic speed (f) A simple clean and conspicuous layout

At all roundabouts, particularly those on high speed roads, traffic is slowed down so that under all flow conditions, vehicles traverse the junction at a safe speed This can be achieved by ensuring that crossing movements at the junction are deflected so as to eliminate straight through paths. It is also important that the layout and signing should make drivers aware of the junction. The need to reduce speed and give way to traffic on left also is important.

The main purpose of this paper is to ultimately design conventional roundabouts, which are commonly used in Nigeria, with a view to improving (her capacity.

LITERATURE REVIEW

O' Flaherty (1974) stated that the main factors controlling the capacity of a conventional weaving section are the geometric layout which includes the entrances and exits and the percentage of composition of the weaving traffic. According to Blackmore (1963). the maximum flow through a weaving section at speeds between 14.5 and 24 km/hr is given by

Qm = (35w(1+e/w)(1-p/3)}/(1+w/l)

Eqn (1)

Where:

Qm = max flow in dry weather pcu/hr

w = width of section (6.1-18.3m)

e = average entry width (0.12w-0.13w)m

p = ratio of weaving traffic / total traffic varying between .4 and 1.0

I = length of section (0.76w-2.5w)

The design capacity should be taken as 80% of the value computed by the equation to avoid overloading of a weaving section during periods of high unstable traffic flow. The possible capacity during wet weather is about 10% lower (Salter. 1978). The above factors should be applied after the following corrections have been made

(1 ) The computed capacity should be reduced by 16% when more than 300 pedestrians per hour cross the exit road from the section being considered

(2) Reduce the calculated capacity by 5% if the existing vehicles have to turn through an angle greater than 75 degrees when leaving the weaving section and by 2.5% if the existing angle is between 60 and 75 degrees (3) reduce the calculated capacity of the preceding section by 5% if the angle of the section being studied is less than 15 degrees. (4) if the angle of entry is between 15 and 30 degrees reduce by 2.5% According to research carried out by Wohl and Brian (1967). it was found that capacity should be taken as 85% of the calculated capacity. They also used the formula below (Eqn. 2) to determine what they called practical capacity QP where QP is given as:

Qp = {160w(1 + e/w)}/ (1 + w/2) vph ...Eqn (2)

This practical capacity is said to be valid under conditions when.

(a) there are standing vehicles on the approaches to the roundabout

(b) the site of the roundabout is level and approach gradients do not exceed 4%.

The formula is based on observations of actual roundabouts. The variables fell within the ranges indicated below: W = 9.1m - 18.0m e/w = 0.63-0.95 w/l =0.38-061 e1/e2 = 0.34 - 1.14

For conventional roundabouts, Wohl and Brian prescribed the following requirements:-

(a) The weaving sections should be adequate for capacity

(b) The central island should be simple geometric figures e.g. circles or ellipses.

c) Entries may be widened for multiple vehicle entry.

d) Prevention of excessive vehicle speed on weaving lengths. (c) Widening on curves should be recommended to assist the average vehicle.

VOLUME STUDIES

Traffic volume studies were carried out at three of the roundabouts in llorin City, capital of Kwara State . These are the Geri Alimi Roundabout. Post Office Roundabout and Jebba Road Roundabout.

The Geri Alimi Roundabout is a four-arm roundabout with a large circular central island.

The Jebba Road Roundabout is a three-arm roundabout with a very large elliptical central island.

The Post Office Roundabout is also a four-arm intersection with a large circular central island

Each of the roundabouts is the conventional type. All the arms of both the Post Office and Jebba Road Roundabouts are dual carriage ways. Only one arm of Geri Alimi roundabout is dual carriage way. The other three are single carriage ways.

Manual counting method was used for the volume studies and every vehicle that passed a fixed point on each arm of the intersections was counted. The vehicles were classified into three groups viz.:

1) light vehicles - cars taxis, motorcycles and light commercial vehicles with four road wheels < 15kN

2) Public service vehicles - buses and coaches of 6 road wheels. 3) Heavy goods vehicles : vehicles having 6 or more road wheels and over 15KN unladen

weight.

Fig. I: Geometric Parameters Studied

Figure 1 shows a section of a roundabout depicting the geometric parameters considered in the write-up. The figure shows only 2 arms of a typical multi-arm roundabout with the entry and exit vehicles channelized. A weaving section is also shown with the weaving length and I and w respectively.

TABLE 1: FLOW FOR GERI ALIMI ROUNDABOUT

Road Arm I e w P Qm QP

From Air port to Asa Dam Road

20 4.75 7.3 0.52 2583 2066

From Asa Dam Road to Umar Saro Road

20 5.11 7.3 0.49 2693 2154

From Umar Saro Road to Adewole Road

18 5.69 7.3 0.95 2219 1775

From Adewole Road To Air Port Road

18 5.84 7.3 0.84 2376 1900

TABLE 2: FLOW FOR JEBBA ROAD ROUNDABOUT

Road Arm I e w P Qm QP

Fate Road to Jebba Road

27 5.32 7.6 0.60 2859 2258

Jebba Road to Muritala Moh Road

77 4.75 7.3 1.00 2695 2156

Muritala Moh. Road

to Fate Road

18 5.84 7.3 0.60 2640 2112

TABLE 3: FLOW FOR POST OFFICE ROUNDABOUT

Road Arm 1 e w P Q... QP

From Emirs Road to Bank Road

34 4.7 7.3 0.58 2837 2270

From Bank Road to Sulu Gamban Road

41 5.7 7.3 060 31 19 2495

From Sulu Gamban Road to Muritala Moh Road

34 4.8 7.3 0.80 2588 2070

From Jebba Road to Emirs Road

30 5.1 7.3 040 3082 2466

TABLE 4: DELAY STUDIES

ROUND- ABOUT ARM OF INTER- SECTION AVG DELAY (Sec./ veh)

TOT. AVG DELAY (Sec./ veh )

Gen Alimi Umar Saro Rd Asa Dam Rd Airport Rd Adewole Rd

4.20 5.70 7.00 9.90

6.70

Post Office Muritala Moh. Rd. Emirs Rd. Bank Road Sulu Gamban Rd

4.20 8.60 9.20 10.70

8.20

Jebba Road Muritala Moh Road Jebba Road Fate Road

5.80 4.20 3.60

4.57

The data collected during the peak periods were used to obtain Tables 1-3 for each of

the roundabouts using Eqn. 3 below. Qmax = 354w(1+e/w)(1-p/3)/ (1+w/L)

...Eqn. (3)

to determine the maximum flow Qmax and then later determine the practical capacity Qp.

From the tables it can be seen that the highest flow occurs at the Post Office Roundabout with 9301 vph. The roundabout with the least volume out of the three studied is Jebba Road Roundabout with hourly volume of 6526 vph.

SPEED STUDIES

Speed studies were carried out at the entry points and at the circular sections of the roundabouts. Speed here means spot speed which is defined as the instantaneous speed of a vehicle at a specif ied location as indicated by the speedometer. This is used to determine the effect of a particular traffic flow constriction such as a roundabout

The mean speed values at the entry points and round the circular portions of Geri Alimi Roundabout are 30km/hr and 18km/hr respectively. For Jebba Road Roundabout, the corresponding values are 45km/hr and 25km/hr while those for the Post Office Roundabout are 28km/hr and 15km/hr respectively.

DELAY STUDIES

Delay to vehicles at roundabouts are of two types.

(a) Delay caused by going down to negotiate the roundabout by traveling the extra distance and by accelerating to normal speed

(b) Delay caused by interaction with other vehicles using the roundabout (i.e. Delay for queuing and weaving action)

Delay studies were carried out on the three roundabouts. Table 4 shows the average delay to all vehicles from each of the approaches. The average delay values for each of the arms of the roundabouts are generally low. The lowest occurring

at Fate Road on Jebba Roundabout i.e. 3.6 sec./veh with max flow of 2859 vph. The highest delay value occurred on Sulu Gambari Road at the Post Office Round about with maximum flow of 2588 v.p.h The Roundabout with the lowest average delay per vehicle was Jebba Road Roundabout with average delay of 4 50secs/veh and total capacity of 6526 vph While the highest average delay per vehicle of 8.20sec/veh was experienced at the Post Office Roundabout with total capacity of 9301 vph This was expected because of the volume of traffic using the roundabout, the proximity to railway crossing and high pedestrian activity

CONCLUSION

Three traffic parameters: volume, speed and delay for three to four roundabouts in llorin town were considered. The capacity of the Post Office Roundabout 9301 vph. was found to be the highest.

This roundabout, i.e. the Post Office Roundabout, recorded the highest delay of 8.2 sec per vehicle With the minimum entry speed (28km/hr) and weaving speed (15km/hr) the Post Office Roundabout poses a potential bottleneck to traffic and motorists alike. Future studies on this roundabout should look at the possibility of widening the entry and exit arms of the roundabout This is with a view of increasing the capacity and further reducing the delay to traffic. It is suggested that other types of roundabout e.g. the small roundabout or the mini roundabout should be introduced on experimental basis for a number of months. It is noteworthy that the roundabouts are cost effective in the sense that none of them needs to be manned by any traffic warden.

REFERENCES

O' Flaherty C.A (1974): Highways & Traffic. Edward Arnold Publishers. London. Blackmore F.C. (1963): Priority at Roundabouts. "Traffic Engineering and Control". Vol. 5. No 2. pp 104-106 Salter R.J. (1978): Highway Traffic Analysis and Design. Macmillan Press Ltd. New York Wohl M . and Brian V.M. (1967): Traffic System Analysis for Engineers and Planners. McGraw Hill BOOK Company. New York