Special provisions for left turns at signalized intersections to increase capacity and safety
✍ Scribed by Bu-Yong Shin
- Publisher
- Institute for Transportation Inc.
- Year
- 1997
- Tongue
- English
- Weight
- 619 KB
- Volume
- 31
- Category
- Article
- ISSN
- 0197-6729
No coin nor oath required. For personal study only.
✦ Synopsis
Heavy left-turns at intersections are not only difficult to accommodate but also often cause traffic accidents. Such problem can be reduced by adopting an exclusive left-turn signal phase, but in this case the cycle length needs to be extended and the overall intersection capacity reduced. This problem is particularly important in such countries as Korea where unprotected left turns are rarely used.
Innovative intersection designs and operational methods for avoiding these problems are presented and their effects of increasing capacity and reducing vehicle delay are analyzed. They rule out the requirement for exclusive left-turn phase while accommodating a large volume of left-turns quite safely.
Why special treatment
At an intersection a vehicle may go straight, turn right or left. A vehicle turning right can do so without interfering other movements, except for pedestrians. Therefore, right turns are normally allowed during all signal phases and seldom cause capacity problems. So, not much provision is required for right turns.
Straight-ahead traffic of east-west bound and north-south bound intersects each other. Left-turns from any approach are unique in that they interfere with traffic in both east-west and north-south directions. Therefore, in order to facilitate a left-turn, traffic on both directions should yield or stop. Since left-turn volumes are normally smaller than that of straightahead movements, often, only one or no lane is assigned exclusively for left-turns. This means that for only one lane of left turn flow, all though lanes on three approaches should be interrupted. This will of course reduce the operation rate of the intersection. i.e. the ratio of operating lanes out of the total approach lanes. For this reason, traffic engineers