How do they do that? is a new series for The Source that explores the technology that helps keep Metro running and passengers and other commuters moving. Some of it applies directly to the trains, buses and freeways and some of it runs in the background — invisible to nearly everyone but essential to mobility in our region.

How do they close down 405 freeway lanes and nearby streets for construction?

There’s a method to what may seem like madness when it comes to closing down freeway and street lanes for the I-405 Sepulveda Pass Improvement Project. And no, it’s not just you who thinks it odd that there often seems to be no one working where the lanes are closed.

Among the reasons the workers seem to be missing, even though orange cones are in place:

— They are invisible because the construction zone is around a curve.

— They are a long way away because safety regulations require a significant amount of space for the cars to slow down from high speeds.

— They are a long way away because safety regulations require a significant amount of space for cars to accelerate from slow speeds.

— They actually are working at another spot but the equipment — think K-rail — is so difficult to move that it may be left in place for a few weeks until the workers return to complete a particular task.

— There are no workers nearby but there is potential invisible danger to cars from above.

Although it might seem random to the harried driver, traffic control in a construction area like the Sepulveda Pass is dictated by the Los Angeles Department of Transportation (DOT) for city streets and Caltrans for freeways and their ramps. The traffic plan incorporates the psychology of drivers and their reaction times with the goal of safety for everyone — driver and worker alike.

For example, on Sepulveda Boulevard north of Moraga a construction zone for a curved street could begin with the tapering of a lane so that motorists will merge into one lane before a curve begins. Tapering begins a long way prior to the work zone. But it could be dangerous to taper the lanes just before the work zone because drivers would have to adjust speed and direction quickly, with limited sight lines.

In another example, one lane of an off ramp may be closed to prevent drivers from two off-ramp lanes having to quickly merge into a single lane of a major street. This happens frequently at the Montana/Sepulveda intersection.

At the crucial Wilshire/Sepulveda intersection, a long tapering of lanes leaving Wilshire is required because safety standards prohibit tapering a lane immediately after a major intersection. (There actually is a table on how long a merge taper must be, including cones and signs facing traffic, based on the speed limit.)

Because of safety concerns, work zones often include empty spaces to create a buffer between workers and live traffic. And for an activity such as pavement saw-cutting, a crew may be moving from one section to another but the entire area must be free of vehicles.

A perfectly good lane might be closed not because of work at street level but because a crew on an adjoining hillside might be clearing vegetation. Safety requires an area for pieces to fall without harming drivers. This situation has prompted closure of the southbound Getty Center Drive ramps.

Construction vehicles, such as large trucks, need a length of lane to accelerate to freeway speed from work areas. This happens frequently near the Skirball and Mulholland bridges.

All these requirements and issues demand hours spent configuring the temporary closures. It’s estimated that Kiewit — the prime contractor for the 405 project — and subcontractors place 1,400 cones on an average night. For night work, eight crews set up an average of 35 closures per night.

So the next time you’re stuck on Sepulveda Boulevard and you’ve already listened to the long version of “Stairway to Heaven,” just think of the patterns at work to keep you and the workers safe. It may offer some comfort or, at least, a needed diversion.

In the meantime, here’s an interactive map of some of the major projects on going along the 405.