How effective are those U-turns in major thoroughfares in Metro Manila? This is the question addressed by a paper entitled “Modeling U-turn traffic flow” appearing in Physica A in 2010 by two Filipino researchers Jay Samuel Combinido and May Lim, both are from the National Institute of Physics, University of the Philippines in Diliman. Using computer simulation, they examined the effectiveness of median U-turns in major thoroughfares in the Metro. These median U-turns are installed in busy intersections to improve the traffic flow by eliminating left turns in these intersections.
Combinido and Lim used a microscopic traffic model to capture the interaction of motor vehicles entering or leaving the U-turns. Using computer simulation, they ran several conditions in terms of the number of cars entering and leaving the U-turn and examined the resulting flow of motor vehicles. Interestingly, two traffic states emerged depending on the density of cars in the U-turn.
For some density of cars below a critical value, the behavior is linear resulting in a free flow state. As the density of cars increases beyond the critical value, the flow of cars reaches the maximum capacity of the U-turn, the road becomes saturated, and the system can no longer handle higher car inflow leading to a congested traffic state.
The results therefore suggest that for the U-turns to be effective, minimal interaction between cars is a necessary condition. For high density intersections where the interaction will be high, U-turns worsen traffic situation instead of minimizing congestion.
You can read the details of the study in “Modeling U-turn traffic flow” by Jay Samuel L. Combinido and May T. Lim, Physica A 389 (2010) 3640 – 3647.