When you’re driving the highways, do you ever wonder why traffic slows down and then speeds up for no apparent reason? Often, it’s not clear why that happens. Sure, it could have been because of an earlier incident, but sometimes it’s not as clear cut as that. Our Operations Center gets asked this question a lot. People will call and say, “After the hill, traffic speeds up, but I don’t know why.” Perhaps the volume of traffic is heavy, but not that heavy. Of course, if an accident or breakdown happens, you can see why traffic is slow. Other times, the reasons are less obvious to the casual observer. Here are some of those lesser-known reasons:

Residual Delays: We receive a lot of calls on this. Drivers say, “I was jammed up to this exit and the traffic just opened up, though there is nothing here.” Generally, that’s a residual delay caused by an earlier crash or breakdown. It takes some time for that traffic flow to filter out once an incident is clear.

Hills: Even a gradual upward incline can, given the proper circumstances, slow traffic flow, especially for those of us driving small, somewhat underpowered vehicles. It can be difficult to maintain highway speed going uphill. If you’ve ever driven Route 128 South in Burlington going uphill near Route 3 you know what I mean.

Solar Glare: We talk about this a lot especially in the spring and fall, when the sun rises and sets at the peak of the rush hour. It’s natural, and safest to slow down and give yourself additional space when your ability to see the car in front of you is limited. Sunglasses help, but don’t solve the problem. Clean windshields, inside and out, lessens the significance of solar glare.

Lane Drops: Mean that the roadway narrows by one lane. The lane drop on Route 93 South by Dascomb Road is an example, as is Route 1 south at Route 99. Of course, lane additions are great for relieving traffic congestion approaching it.

Merging Volume: Any heavily populated area can handle additional on and off ramp volume, which can result in delays. The best example of this is on the Neponset Circle onramp to the NB Xway. During the afternoon, it’s not unusual to have delays approaching that on-ramp. Then, once past the ramp, traffic improves.

Curves: Can also slow traffic if the driver is unsure whether traffic has backed up around the corner.