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Drivers in cities who encounter a new sign prohibiting a left turn or a roundabout or a motor vehicle diverter might be able to blame navigation apps for that obstacle.

While apps like Waze and Google can save drivers time and aggravation, they can also cause headaches for the people in charge of designing cities’ transportation infrastructure.

鈥淓ssentially you have different goals,鈥� said Eric Shimizu, principal engineer and regional director for smart cities and connected vehicles at DKS Associates. App developers 鈥渁re into computer analyses and how to help people with flexibility and making choices. We have to worry about designing safe transport.鈥�

Engineers responsible for ensuring safe and efficient travel on city streets are adopting a variety of strategies to accomplish their goals. They can’t tell apps where to direct drivers, but they can influence traffic enough to make their desired outcomes favorable to the apps.

When an app tells a driver to travel on a residential street, it is simply relying on an algorithm that determines the most efficient route. A traffic engineer has to also take into account the likelihood, especially on smaller streets, that a cut-through driver will exceed the posted speed limit and consequently increase the chances of a fatal accident.

The chances of a pedestrian fatality in a collision with a vehicle nearly double as the vehicle’s speed increases above 25 mph, according to study results from the AAA Foundation for Traffic Safety.

鈥淭ypically, when we look at projects, the big arterials are where we want the traffic to go,鈥� said Hermanus Steyn, senior principal engineer at Kittelson & Associates. 鈥淲hat is happening is (the apps) look at the whole network. We want people to travel Barbur (Boulevard). With the apps, they are trying to find the quickest routes between points A and B. The big models we use don’t include the smaller routes.鈥�

The end result is that apps often send large numbers of motor vehicles to streets that weren’t designed to handle such volume.

Engineers are beginning to take this into account. During a recent traffic mitigation project in Clyde Hill, Washington, Shimizu said, consideration of how apps would react to changes in the streetscape became a major part of the project.

DKS’ studies found that nearly half of drivers in the Seattle suburb used navigational apps. Due to congestion on State Route 520, apps sent a lot of drivers to smaller neighborhood streets. This was despite the presence of an available arterial, Bellevue Way.

鈥淏ellevue Way was underutilized for how big the road is,鈥� Shimizu said. 鈥淚t could handle the increased capacity, but the apps were telling drivers that the cut through is saving time.鈥�

Engineers’ solution hinged on increasing the drive time along cut-through routes during rush hour. Their traffic studies revealed that when drive times through the most popular cut-through routes increased by two to three minutes, the apps would send drivers back to the arterial roads that were designed to handle the volume.

鈥淲e calculated times over the course of nine months,鈥� Shimizu said. 鈥淲hen we were doing this study, (drivers) would save three minutes, which is significant on a short trip of about 20 to 25 minutes. The apps are pretty good in saving time.鈥�

Engineers adopted a multiphase strategy to influence behaviors of both navigational apps and drivers. First, they made a major effort to inform regular drivers of the speed limits on the smaller roads.

鈥淭hey started slowing down,鈥� Shimizu said. 鈥淲e had a good timing benefit. The apps take the actual driver information it is collecting. It knows what velocity drivers are traveling at. That information and that calculation formulates the path routing.鈥�

The next step included a combination of strategies. They added no-left-turn signs on numerous streets to force drivers to travel past signals and then come back to make a right-hand turn. They also added permanent roundabouts on numerous east-west streets to slow drivers even more.

Lastly, they tweaked traffic lights that grant access to State Route 520.

鈥淭hey adjusted the meter rate just slightly, which added the minute and pushed us over the two- to three-minute goal,鈥� Shimizu said.

Engineers largely got what they wanted. As traffic slowed on the smaller surface streets, the apps began routing drivers back onto the larger roads.

鈥淎ll the sudden we saw a big shift to go to Bellevue Way,鈥� Shimizu said. 鈥淚t has lanes and plenty of space. We saw with the apps that eight to nine out of 10 drivers avoided the cut through.鈥�

Traffic engineers in Portland are using similar techniques, although few go to the extent that designers did in the Seattle-metro area.

鈥淧ortland is one of the test cities to reduce speeds to 20 mph from 25 mph,鈥� Steyn said. 鈥淏asically the idea is to target speed if you can’t control the algorithm.鈥�

The new speeds will roll out next year, Portland Bureau of Transportation Public Information Officer Dylan Rivera said. PBOT officials hope the traffic apps will consider a 20 mph street less attractive.

鈥淲e think Waze would consider those residential streets with a 20 mph speed limit as less favorable alternatives to busy streets that are marked 30 mph, for example,鈥� he said. 鈥淭oday they might see a residential street with a 25 mph speed limit as a viable alternative to a busy street.鈥�

Planners also must deal with unintended consequences. An effort to limit cars on one street to promote bicycle and pedestrian use can send a stream of vehicles down another parallel street that can’t handle the volume.

When city planners aimed to increase bicyclists’ use of Southeast Ankeny Street, they faced a big challenge: it runs parallel to Southeast Burnside Street. When Burnside is crowded during rush hour, drivers flood onto Ankeny. Although no study was performed to identify how many drivers were rerouted there by apps, engineers knew they had to direct them elsewhere.

鈥淥ur policy is Ankeny is a greenway and to prioritize it for bike use,鈥� Rivera said. 鈥淲e have put in diverters that force traffic off Ankeny.鈥�

The diverters (physical, man-made barriers) allowed bicyclists to roll though, but not drivers. Early studies showed a 20 percent reduction in the traffic on Southeast Ankeny; it wasn’t 100 percent because some apps won’t show diverters and direct drivers onto streets blocked to vehicular traffic in reality, but appear clear in cyberspace.

Another issue is that drivers often trust their technology more than their own eyes. When the no-left-turn signs were posted in Clyde Hill, drivers frequently would turn left anyway because their apps said they could.

鈥淭he cops would sit there and were pointing at them to don’t (turn),鈥� Shimizu said. 鈥淭he driver would look at the app and make the turn anyway.鈥�

While navigation apps can hurt an engineer’s best-laid traffic plans, they can also provide useful information.

鈥淥verall, they have been more a benefit than detriment,鈥� Shimizu said. 鈥淕oogle alone has helped do field reconnaissance. Initially, you get a feel of what is out there. We love the added information.鈥�

Meanwhile, PBOT officials are engaged in ongoing conversations with Waze developers.

鈥淲e are trying to figure out how to get their app to show school zones,鈥� Rivera said.