Are additional sensors needed for Waymos?

When well over 1” of rain fell rather quickly in various parts of Phoenix recently, areas prone to flooding accumulated much more. As one of the highest amounts of rain to fall within the past seven years, it provided a new scenario to Waymo robotaxis. Several of the driverless vehicles confronted “Do Not Enter When Flooded” signs, stopped with riders inside, and did not proceed to their targeted destinations.

Operating as a rider-only service in the Phoenix area since 2020 as Waymo One, Waymo is now open across 315 square miles of the Valley of the Sun. The flooded areas provided a new challenge to Waymo engineers. The immediate solution was to suspend service. For the long term, other options are needed.

Sensing water depth

Figure 1. A flooded roadway stops a driverless Waymo vehicle in its tracks. (Image: KTVK/KPHO)

Sports utility vehicles (SUVs) designed for off-road duty frequently encounter water. The depth of water (wading depth) a certain vehicle make/model can handle is usually specified by the car maker. For example:

With a best-in-class ground clearance, the Ford Bronco has a wading depth of 33.5 inches.
The Toyota Land Cruiser has a wading depth of up to 2 feet.
Wading depth on a Jeep Wrangler is 30 inches.
The Range Rover has a ground clearance of nearly two feet and a wading depth of almost three feet.

However, other factors, such as traction and rate of water flow, impact a vehicle’s capability to wade. Since 2014, Land Rover has offered the Wade Sensing system to measure the water depth around certain vehicles. Sensors mounted in the vehicle’s mirror housings use ultrasonic sensing technology to emit sound waves that bounce off the water’s surface and return to the sensor. By measuring the signal’s round-trip transmission time, the SONAR system accurately calculates the water depth. When the driver selects Wade Mode, it raises the vehicle’s suspension height to reach its maximum wading depth and ford water depths of up to 900 mm (35.4 inches). Designed as part of Bosch’s driver assistance platform, the platform also has a 3D Surround Camera System to view the vehicle’s immediate perimeter using four wide‑angle high definition (HD) cameras, each providing a 190‑degree field‑of‑view, as well as 14 ultrasonic sensors.

Figure 2. The infotainment’s touchscreen display provides the interface for wade sensing. (Image: Land Rover)

Moving forward

During its initial exposure to Phoenix area flooding, Waymo suspended service in the area for a period of time. Two weeks later, when even stronger rainfall occurred, Waymo’s response was apparently successful since no reports of stuck vehicles occurred, or if they did, they did not register the same response in the local and social media as the previous rainfall.

However, sensing for water depth is not a solution to rapidly flowing water that can occur during flash flood situations. When these floods occurred in the past with any type of vehicle, motorists who were willing to try and cross the flooded areas were surprised at the depth they encountered and discovered that as little as 2 feet of rapidly flowing water can quite easily float a vehicle and carry it downstream.

Areas where flooding has previously occurred often have “Do Not Enter When Flooded” signs. In these cases, there is no sensing technology (except cameras through Traffic Sign Recognition (TSR) technology) that can substitute for reading and following the signs. Unlike human drivers who try to cross the identified dangerous areas, it looks like the driverless vehicles made the right decision in the first place.