Left-turn crashes are among the most frequent and dangerous types of collisions on U.S. roads, especially when a driver turns across the path of an oncoming vehicle. Known formally as Left Turn Across Path – Opposite Direction (LTAP-OD) crashes, these incidents are usually sudden, difficult to predict, and challenging to evaluate after the fact.
To better understand how drivers respond in these high-risk situations, researchers from the Driver Research Institute (DRI) turned to the SHRP-2 Naturalistic Driving Study: one of the most detailed collections of real-world driving data ever compiled. It includes continuous video, GPS, and in-vehicle sensor data from over 3,500 drivers across the country.
For this analysis, researchers reviewed 122 LTAP-OD events. Of those, seven were left-turn collisions, and 115 were categorized as near-crashes.
What Is an LTAP-OD Crash?
A left turn across path crash occurs when one vehicle turns left across the path of a vehicle traveling straight from the opposite direction. They usually take place at intersections, but they can also happen when a car pulls into a driveway or business entrance from across the road.
They’re one of the most common and deadly types of crashes involving young drivers. On the surface, they may seem preventable. After all, the turning vehicle is usually visible. So why doesn’t the oncoming driver simply brake or swerve in time?
That’s the question this research set out to answer.
The Goal of this Analysis
The researchers wanted to better understand how long it takes drivers to respond when a car turns left in front of them. That includes:
- When they first noticed the turning vehicle
- How quickly they braked or swerved
- Whether factors like lighting, weather, or distractions changed driver reaction times
Researchers also examined a common assumption in crash investigation: that drivers should react within 1.5 seconds. The findings show that real-world critical crash response time can vary, and that assumption may not always be realistic.
When Do Drivers Start Reacting?
The researchers explored four different moments that could mark the beginning of a driver’s response to a left-turning vehicle. These “starting points” helped them measure when drivers recognize a threat and begin taking action:
- When the turning vehicle (called the “principal other vehicle” or POV) first becomes visible
- When the POV begins moving laterally across the road
- When the POV enters the driver’s closest lane
- Time-to-conflict, or how many seconds remain until impact if both vehicles keep their current speeds
The data showed that drivers most often reacted during moments 2, 3, and 4. Just seeing the turning car wasn’t enough to prompt an emergency maneuver. In some scenarios where the vehicle was visible for 15 seconds, the response times were over 11 seconds, thus not providing useful results.
How Quickly Did the Drivers Respond?
The answer depends on the behavior of the turning car.
- If the vehicle was already moving and did not stop first, drivers typically responded within 1 second.
- If the vehicle had been stopped and suddenly began turning, the average response time increased to 1.6 seconds or more.
The delay may be because drivers assume the stopped car will wait for a safe gap. When it suddenly turns, it catches them off guard.
What Influences Reaction Time?
Not all situations lead to the same type of response. Several factors were found to affect left turn crash reaction time:
1) Time-to-Contact and Distance
Drivers reacted more quickly when there was less distance or time before a potential collision. Urgency seems to be a major trigger in quick decision-making.
2) Obstructed View
If another vehicle blocked the driver’s line of sight and the turning vehicle appeared suddenly, reaction time decreased. The element of surprise likely forced a faster response.
3) Number of Lanes to Cross
The more lanes the turning car had to cross, the earlier drivers started to respond. Researchers think this may be because longer lateral movement gives more visual cues that the car is turning.
Did Drivers Brake Hard Enough to Avoid a Crash?
Not always.
The study measured whether drivers reached 0.4g or more in deceleration, which is a sign of emergency braking.
In some events:
- Drivers did not immediately react to the turning vehicle
- They did start reacting, but didn’t brake hard or quickly enough
The hesitation typically occurred when the turning car was visible but didn’t initially seem like a hazard. It shows that seeing a car doesn’t mean the brain instantly registers it as dangerous.
Surprising Findings About Road Conditions
One unexpected finding was that lighting and weather didn’t significantly affect driver reaction time in left-turn crashes. Drivers reacted similarly whether it was daylight or dark, clear or rainy.
However, location did make a difference. Drivers were slower to respond when the turning vehicle entered from a mid-block driveway or business entrance, rather than a standard intersection. This suggests that drivers expect intersection accidents, but are less prepared for turn across path crashes that happen in unusual places.
What This Means for Crash Investigators and Vehicle Designers
The study offers several important takeaways for professionals involved in crash analysis and vehicle safety design.
Crash investigators and reconstruction experts now have strong evidence that drivers don’t always react the instant a threat appears. Driver reaction time varies based on the timing of the turn, whether the turning car stopped first, whether it was obscured, and where it was turning from. A rigid 1.5-second rule doesn’t always reflect real-world behavior.
For vehicle designers and ADAS engineers, the findings highlight the need for smarter systems. If an automatic emergency braking (AEB) system waits as long as a human driver might, it could miss the chance to prevent a turn across path collision. Anticipating risk based on typical driver behavior (rather than perfect reactions) can lead to better safety outcomes.
What About Court Cases or Legal Use?
These insights also carry significant weight in legal settings
It’s common to hear claims like “the driver had enough time to react.” However, the study clearly shows that’s not always accurate. Just because a vehicle is visible doesn’t mean a driver will instantly perceive it as a threat. The full context matters.
- Was the car simply moving, or had it just started turning?
- Was it partially hidden at first?
- Was it turning from an unexpected location, like a business driveway?
- Were there any clues that a turn was about to happen?
With this research in hand, crash experts can offer a science-backed explanation for why a driver may have been unable to avoid impact, even when surface-level details suggest otherwise.
Ready to Go Deeper?
If you’re a crash investigator, attorney, or expert witness, understanding how and when drivers respond in these TAP-OD scenarios can change the way a case is interpreted. You now have peer-reviewed data showing that not all drivers react in the same way, and that expecting uniform response times is not backed by real-world evidence.
The Driver Research Institute helps law enforcement, reconstruction experts, vehicle engineers, legal teams, and insurers apply real driver behavior to high-stakes questions. With validated tools like Response and science-backed analysis grounded in naturalistic driving data, our team brings clarity to cases where timing, visibility, and human behavior are critical.
Looking for expert accident reconstruction or human factors support? Contact us today to put proven research to work in your next investigation or courtroom presentation.
