The Effectiveness of Control of Speed by Illusion Lines (COSBI Lines)

Abstract

The case study is on a section National Route (N2) near Somerset West/Strand, South Africa. The N2 is a divided dual carriageway surfaced freeway from Cape Town to the start of the experimental section located at the end of the freeway. The road changes from west to east from a high-speed zone to a low-speed zone, leading into two left and a right curves with a priority controlled intersection between the left and right curves. The change from a high- to a low-speed zone results from a change in road classification from a freeway posted at 120 km/h to urban arterial posted at 60 km/h. The geometry approaching the curves from the west is a 2 km straight section of freeway. The geometry allows the motorist to travel at speed around the curve but it is a safety risk due to the reverse curves and the intersection between the curve, as well as queuing in the afternoon peak periods due to the set of traffic signals 1,44 km to the east. Control of Speed by Illusion (COSBI) lines are 600 mm wide transverse painted markings (bars) that are set out at a gradually decreasing spacing that gives the motorist the illusion that he is increasing his speed. This is known as a set of bars. The spacing of the COSBI lines was calculated to reduce the speed of the motorist from 120 km/h to about 60 km/h. It would be normally recommended that the road marking material be thermo-plastic or cold melt plastic to ensure that there is a slight level difference. With the level difference, the bars also act as a rumble strip. For this case study the use of normal retro-reflective road marking paint was used as the road is to be resurfaced. The South African National Road Agency SOC Limited (SANRAL) approved, the case study to determine the effectiveness of COSBI lines by performing a speed study before and after the application thereof. Speeds were measured by means of a radar gun. The disadvantage of using a manual speed measuring device is that the speeds of only isolated vehicles or the first vehicle in a platoon can be measured. This was not considered a severe constraint as both the before and after studies were done under the same constraint and the single or leading vehicle would be the speeding vehicle of which the behaviour needs to be monitored and changed. The case study found that a 10 to 15 km/h speed reduction was achieved with before speeds of up to 87 km/h reducing to 72 km/h over the short term as measurements were made about the day after and 5 weeks after installation. It is recommended that longer term after studies be done to check if drivers revert to previous speeds. The design procedures formulated by Katz (2007) is not applicable to local conditions.