Drone footage shows Highway 101 southbound traffic slowing near Rengstorff Ave, where ramp meters remain off. Free-flowing on-ramp traffic merges at a 1:1 ratio, creating waves of congestion. Caltrans should investigate why the meters aren’t working.

Executive Summary

Over the past 3 years, I've noticed a consistent slowdown on all Highway 101 southbound lanes for around 1.5 miles between Rengstorff Ave and Oregon Expressway in Mountain View and Palo Alto.

Using drone footage, I observed that the problem seemed to be exacerbated by a stream of free-flowing vehicles unregulated by ramp metering lights at both Rengstorff Ave southbound on-ramps. I noticed that both ramp metering lights were off during evening rush hour.

I recommend that CalTrans investigate why the ramp metering lights are off and consider turning them on.

The Metropolitan Transportation Commission states that "ramp metering traffic signals on freeway on-ramps reduce freeway congestion by controlling the frequency at which vehicles enter the freeway, and preventing large groups of vehicles from entering all at once" (Adaptive Ramp Metering, MTC).

Context

Over the last few years, I've consistently noticed that the Highway 101 traffic slows down significantly around Mountain View - between the Highway 85 and Oregon Expressway - in both directions. In this blog, I will focus on the southbound direction.

It's a very consistent slowdown; it happens every day of the week during evening rush hour between 3pm - 6pm. Finally, I decided to fly my drone last week to try to get to the bottom of the issue.

In the drone video above, the drone is facing southeast and we see the Highway 101 Rengstorff interchange. I noticed that there was a slowdown in traffic (off-screen, behind the drone) in the southbound direction (right side of the highway) leading up to the interchange. It seems to be attributable to 2 main causes: lack of traffic metering at the Charleston Road on-ramp and lack of traffic metering at the Rengstorff Ave southbound on-ramp.

Here is the full video:

Ramp Metering Light Not Functioning

Charleston Road On-Ramp

Cars from Charleston Ave proceed southbound without any stop sign and merge directly onto the highway.

Between 0:00 and 0:50, 15 vehicles enter Highway 101 from Charleston Road going south. The observed 15 vehicles in 50 seconds is a rate of 18 vehicles per minute.

The merging onto the highway creates congestion because of alternation between 1 car on the highway to 1 car that entered the on-ramp.

Screenshot from 0:17 in the video

Rengstorff Avenue On-Ramp

Between 1:14 to 1:59 in the video, 19 vehicles - which had made a left from Rengstorff Avenue onto the Highway 101 southbound on-ramp - pass the on-ramp metering limit line (white horizontal line). The observed 19 vehicles in 45 seconds is a rate of 25 vehicles per minute.

The merging onto the highway creates congestion because of alternation between 1 car on the highway to 1 car that entered the on-ramp.

Screenshot from 1:45 in the video

Cyclical On-Ramp Merging Causes Highway Slowdowns

The number of cars entering on each on-ramp is cyclical over time because they are dependent on the upstream lighting patterns. Although I am not a traffic engineer, the cyclical merging patterns appear suboptimal for maintaining steady highway flow. There are ~40 second periods of merging at a roughly 1:1 ratio followed by periods of less merging. Anytime the highway slows down, it triggers wave-like effects for miles upstream.

I believe it would be better to have on-ramp metering which could make the flow of cars merging onto Highway 101 more consistent.

Potential Reasons for On Ramp Metering Being Off

Malfunction

I had a discussion with a City of Mountain View public works engineer. They mentioned that there have been issues with a Highway 85 ramp metering light being out, and after multiple reports and some fixing, the lights continue to be out. Based on this data point, it is possible that there could be a similar malfunction at the Rengstorff Ave interchange on-ramps.

On April 8, 2025, Caltrans began installing an Adaptive Ramp Metering (ARM) system on the 101 between Sunnyvale and Redwood City (source); the Rengstorff Interchange in Mountain View falls under this first phase. I have reached out to CalTrans to figure out the status of this project.

Vandalism

Just look at how many streetlights in the CalTrans right of way are out on highway on-ramps. I've seen entire bridges in Palo Alto over Highway 280 with all street lights out. The street lights for the University Ave NB on-ramp to Highway 101 have been out for a long time, even after multiple reports from City of East Palo Alto public works staff to CalTrans. The City of San Jose has had thousands of lights with copper stolen. I am not sure if copper theft is at play here, but it very well could be a contributing factor.

Intentional Off

A connection I have at the Metropolitan Transportation Commission mentioned that typically CalTrans requires the approval of the local jurisdiction to turn on on-ramp metering lights because there is risk of cars backing onto the road. Given the proximity of Rengstorff to Costco and Google, it is possible that Rengstorff Ave could back up if on-ramp metering lights were turned on. You could imagine that the City of Mountain View may have an incentive to prioritize clearing out their local streets over maintaining the flow of traffic on Highway 101.

During my conversations with a Mountain View public works engineer though, they were not aware of any such prohibitions of on-ramp metering lights in Mountain View. More likely, the lights may have become malfunctioning.

Conclusion

Traffic slowdowns on Highway 101 southbound between Rengstorff Ave and Oregon Expressway appear directly tied to non-functioning ramp meters at the Charleston Road and Rengstorff Ave on-ramps. Without active metering, vehicles merge in disruptive bursts at a 1:1 ratio with mainline traffic, creating recurring waves of congestion that ripple for miles. Drone footage and measured vehicle rates suggest the issue has persisted for years. Caltrans should promptly investigate and restore ramp metering to stabilize highway flow and reduce daily gridlock.