TOWARDS THE IDEAL ROUNDABOUT DESIGN - PowerPoint Presentation

Slide1

TOWARDS THE IDEAL ROUNDABOUT DESIGN

C KROGSCHEEPERS

, M WATTERS,

CR

TICHAUER,

FH

VAN

RENSSEN,

G MATTHEESlide2

Content

Background

Review of Existing Roundabouts

– Sample of 37

Inscribed Circle Diameter

Circulatory Lane Width

Entry Lane Width

Truck Aprons

Fastest Paths

Road Signs

Road Markings

ConclusionsSlide3

Why are we still looking at roundabouts?

Confusion?

Inconsistency?

Guidelines?Data?Slide4
Slide5
Slide6
Slide7
Slide8

REVIEW OF ROUNDABOUTS: SAMPLE of 37

Summary

of Roundabouts investigated

Urban

Rural

Single

Double

Single

Double

City of Cape Town

11

11

George

4

1

1

Other small towns

3

411

Part of the process to develop a Roundabout Guideline for Western CapeSlide9

Process: What did we do?

Reviewed Geometric

elements:

ICD, CID, Aprons, Lane

widths,

Gradients

at various locations on the

approaches, Sight

distances,

Entry angles,

Splitter island lengths and widths, Road markings and Signs, etc.

Did Speed measurements

(approach, entry, circulatory and exit positions)

Visual assessment Operations, accident evidence (broken glass, damaged kerbs, damaged signs, etc

.)

Interviews with agency.Slide10

Inscribed Circle Diameter (ICD)Slide11

Inscribed Circle Diameter (ICD)Slide12

Circulatory Lane WidthsSlide13

Circulatory Lane Widths

Circulatory lane widths

. Widths vary considerably. At some single lane roundabouts, the width was greater than 8m resulting in too small a central island diameter. When coupled with a poorly designed truck apron (no vertical separation) the advantage of controlling speed through the roundabout is compromised.Slide14

Entry Lane

WidthsSlide15

Entry

Widths

Entry lane width

. Too wide entry width negates any benefit achieved through the correct design of other elements resulting in too high entry speed. In instances where the wide entry width is coupled with a wide circulatory lane, the measured speeds through the roundabout often exceeded 60 km/h.Slide16

Truck ApronsSlide17

Truck Aprons

The

purpose of the truck apron clearly misunderstood by

someTruck apron often

considered

cosmetic rather than necessary.

External

truck

- useful

- have a positive effect on the entry speeds.

Truck

apron not serving the purpose of designSlide18

Truck Aprons

Truck apron with correct design for use by trucks only with a narrow circulatory roadway

Truck apron with better design, but too wide circulatory laneSlide19

External Truck ApronSlide20

Fastest Path

The principles are

generally

ignoredSlide21

Lordswalk

Fastest PathSlide22

Brighton Fastest PathSlide23

Sunningdale Fastest PathSlide24

Road Signs

The variation in the use of road signs and marking reflects either the lack of knowledge, or the lack of attention to detail or the lack of clear guidance (within the SADC-

RTSM

) for the implementation of road signs and road markings. Slide25

Road Markings

The variation in the use of road signs and marking reflects either the lack of knowledge, or the lack of attention to detail or the lack of clear guidance (within the SADC-

RTSM

) for the implementation of road signs and road markings. Slide26

Conclusions

Lack

of a clear consistent design

standards

ICD generally

within

guidelines

, entry and circulatory lane widths are generally too wide.

Potential

for too

high speeds - compromise in safety.

Pedestrians: No attention - unsafe conditions.

Truck aprons: Not used for the purpose intended.

Road signs and markings: Confusion

Many

variables

influence speed

and

safety. Correct

application of all

elements

are vital

for a safe design. Slide27

Conclusions

Could designs contribute

to the perception that roundabouts are not safe?

Assessment of 37 roundabouts suggest:

Yes.

Both

in terms of vehicle only

and

ped

crashes.

Clearly begs for design standards to provide safe and efficient designs.

Update SADC-RTSM for roundabouts

.