When experiencing a **respiratory allergy** and a sneezing fit, the driver's ability to maintain **concentration in traffic decreases**. This is due to several factors:

- **Frequent sneezing**: During a sneezing fit, the driver may involuntarily close their eyes, reducing their ability to see the road and react in time to any situation.

- **Respiratory discomfort**: Respiratory allergies can cause nasal congestion and difficulty breathing, which can distract the driver and affect their ability to concentrate.

- **Recommendations for drivers with allergies**: Drivers with allergies are advised to take measures to minimize symptoms while driving, such as keeping windows closed, using mild air conditioning, and avoiding driving in conditions that may worsen the allergy, such as at dawn or in humid areas.

These measures and the nature of allergic symptoms indicate that the driver's concentration ability is negatively affected during a sneezing fit.