A substantial majority, exceeding 60%, of vehicular incidents occurring at intersections are attributable to left-hand turns. Consequently, certain municipalities across the United States, such as San Francisco, Salt Lake City, and Birmingham, Alabama, are implementing restrictions on these maneuvers.
Dr. Vikash Gayah, a distinguished professor of civil engineering at Penn State University and the acting director of the Larson Transportation Institute, elaborates on the factors contributing to left-turn-related accidents, traffic congestion, and increased fuel consumption.
The Conversation, in partnership with SciLine, presents abridged and refined excerpts from their dialogue.
What is the magnitude of the danger associated with left turns at intersections?
Vikash Gayah: Executing a left turn necessitates traversing lanes of oncoming traffic. Under a green signal, drivers must identify a sufficient gap before proceeding. An erroneous judgment of this gap can result in a collision with, or being struck by, oncoming vehicles—a scenario known as an angle crash, among the most perilous types of accidents.
Furthermore, drivers intending to turn left are typically focused on the approaching traffic. Unbeknownst to them, pedestrians might be crossing the path of their intended trajectory. This oversight frequently leads to drivers failing to observe pedestrians, which can precipitate severe accidents.
In contrast, right turns involve merging into existing traffic flow but do not directly conflict with opposing vehicles. This fundamental difference renders right turns considerably safer than their left-turning counterparts.
Could you provide statistical data highlighting the specific risks of left turns?
Gayah: Approximately 40% of all vehicular crashes occur at intersections. Of these intersection-related incidents, 50% result in serious injury, and 20% are fatal.
A significant proportion, around 61%, of intersection crashes involve a left-hand turn. Considering that left-hand turns are typically less frequent maneuvers at an intersection compared to other movements, this 61% figure is notably high.
Why do left turns impede the efficiency of traffic movement?
Gayah: When vehicles undertaking left turns are compelled to wait for an opportune gap in traffic, they can obstruct other lanes, particularly when a queue of left-turning vehicles forms.
Many intersections utilize a green arrow signal for left-turning vehicles instead of a solid green light. However, this dedicated phase necessitates the cessation of all other traffic movements. Halting all other traffic solely to accommodate a limited number of left turns significantly compromises the intersection’s overall efficiency.
Additionally, each transition to a new traffic ‘phase,’ such as the green arrow, involves a brief interval during which all signals are red. Traffic engineers refer to this as an all-red time, a period when the intersection serves no vehicles. This all-red duration, typically two to three seconds per phase change, accumulates rapidly and further diminishes intersection efficiency.
What types of restrictions have been implemented in various cities?
Gayah: In less congested urban cores, especially during off-peak hours, permitting left turns is generally acceptable as it doesn’t impose significant demands on vehicle throughput at each intersection.
Some cities have introduced signage prohibiting left turns during specific peak periods, such as 7 to 9 AM for morning rush hour or 4 to 6 PM for the evening commute. For instance, on Van Ness Avenue in San Francisco, left turns are restricted during these peak times.
However, these restrictions are not being adopted on a broad scale. Instead, they are typically confined to individual traffic corridors or isolated intersections rather than encompassing entire downtown areas where their impact could be more substantial and beneficial for the street network’s efficiency.
Roundabouts represent one strategy for circumventing left turns.
Gayah: Roundabouts enhance safety by eliminating the need to cross opposing traffic streams. All vehicles navigate in the same rotational direction. Drivers identify their desired exit and proceed accordingly.
Nevertheless, general left-turn restrictions tend to be more efficient. Roundabouts can become less effective during periods of high traffic volume, potentially leading to gridlock where no vehicles can proceed. Conventional intersections are less susceptible to such gridlock scenarios.
Moreover, roundabouts require a larger spatial footprint. Their installation may necessitate the expansion of existing intersections, which in urban centers could involve the demolition of buildings or the removal of pedestrian walkways. In contrast, restricting left turns merely involves posting a sign indicating “no left turns” or “no left turns during peak periods.”
What are the advantages of prohibiting left turns in urban environments?
Gayah: Regardless of the perspective, the elimination of left turns will invariably lead to increased travel distances. Individuals will need to traverse longer routes to reach their destinations. The extreme outcome involves circumnavigating the block, effectively adding four block lengths to a journey.
However, not all trips necessitate such extensive detours. Within a typical downtown area, the average increase in travel distance per trip is approximately one block length. This marginal increase in distance is largely offset by the enhanced vehicle throughput at each intersection where left turns are banned. Consequently, drivers experience reduced waiting times at intersections on average, leading to a quicker overall journey despite the slightly longer travel distance.
Does avoiding left turns contribute to improved fuel efficiency?
Gayah: Our investigations revealed that despite an increase in average travel distances with restricted left turns, vehicles consume less fuel—approximately 10% to 15% less per trip—due to a reduction in the number of stops at intersections.
This principle underlies the routing strategies employed by logistics companies like UPS, which aim to minimize left turns for their vehicle fleets, thereby reducing idling time and the frequency of stops.
Do you foresee the widespread adoption of left-turn bans?
Gayah: As a relatively novel strategy, it may initially present challenges for some individuals. However, once the benefits of reaching destinations more rapidly are experienced, I believe public acceptance will grow.
View the complete interview for further details.
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