Generally, traffic on the bridge is very heavy at/during rush hour. Last year, traffic on the bridge was (generally) very heavy at/during rush hour. Yesterday, traffic on the bridge was very heavy at/during rush hour. Today, traffic on the bridge has been very heavy at/during rush hour. (reported during or soon after rush hour) Today, traffic on the bridge was very heavy at/during rush hour. (reported later in the day) Today, traffic on the bridge will be very heavy at/during rush hour. (reported earlier in the day) Tomorrow, traffic on the bridge will be very heavy at/during rush hour. In the future, traffic on the bridge will be very heavy at/during rush hour.