You never see marching ants caught in a jam and researchers have some clues why:
#1 : the average speed of ants remains constant, regardless of the density of the traffic.
ants never overtake. Not ever. Instead they form into platoons in which all the ants move at the same speed. Increase the density of ant traffic and the platoons simply join together to form larger groups. This is how the velocity remains the same while the density increases. That makes ant traffic significantly different from other types of traffic in which congestion occurs, such as road traffic and internet packet traffic.