Which of the following is the most common cause of car-bicycle crashes involving motorists?

The most common cause of car-bicycle crashes involving motorists is when motorists turn left in front of oncoming bicycle traffic. This situation often arises due to visibility issues and the different speeds at which bicycles travel compared to motor vehicles. Cyclists may be difficult for drivers to see, especially when they are approaching an intersection.

When a motorist attempts to turn left without accurately judging the speed or distance of an oncoming cyclist, it can lead to severe collisions. Additionally, some motorists may misinterpret the presence of a cyclist, assuming they have enough time to complete the turn safely, which ultimately results in an accident. Understanding this risk emphasizes the importance of both cyclists and motorists being vigilant and aware of their surroundings, particularly in intersection scenarios where such crashes are more likely to occur.

The other choices, although contributing factors in various crash scenarios, do not represent the leading cause as identified in this context. For instance, speeding is certainly a concern, but it does not specifically correlate to the unique dynamic of left turns and bicycle visibility. Texting and forgetting to signal are also crucial distractions and lapses in responsibility, but the mechanics of a left turn directly involve misjudging bicycle traffic and resulting in collisions more frequently.