Alma Avenue is very narrow at the Churchill intersection. The lanes there are now only 9 feet wide; a very tight squeeze. Fitting the Partial Underpass into that space will require some creativity.
Constructing the Partial Underpass will certainly cause disruption of traffic. There is some hope that this can be minimized.
Some of these issues are explored below.
Turning Radius
Large trucks and buses use the Alma/Churchill intersection: over one hundred a day access Palo Alto High School. There needs to be enough room in the Partial Underpass for these vehicles to turn onto and from Alma Ave. The required turn radius is 40 feet.
One way to accommodate this is to add a wide shoulder to the turn lane (the northbound Alma lane). This adds more width to the intersection which is already straining at the limits.
Another technique is to use staggered stops, providing space for a more gradual turn, as in the image below.
Northbound Alma Surface Lanes
The original proposal planned a single northbound Alma lane at the surface, with the left through lane and left turn lanes dropping down to the underpass. It was pointed out by the traffic engineers that there were two problems with this:
- Splitting the two through lanes, one up and one down, will cause confusion for drivers approaching the intersection requiring heightened attention and likely leading to accidents.
- A fire regulation requires a 20 foot minimum roadway in front of buildings to allow the fire department to have two engines, side by side, to fight the fire.
The result is that both northbound Alma lanes must stay at the surface and only the left turn (onto Churchill) should drop down to the underpass. All of the images of the Partial Underpass on this website include this change. Below is a bird's eye view looking north along Alma. It shows the two lanes remaining at the surface through the intersection.
Bike & Pedestrian Safety
Once the number of northbound surface lanes is increased to two, the question of bike and pedestrian safety gains importance. The original proposal had a bridge over the underpass, a tunnel under Caltrain, and a crossing of one northbound lane of Alma.
The proposal on this website has been changed to move bikes and pedestrians to a tunnel at Kellogg. This will go underneath Alma and the Caltrain tracks, separating bikes and pedestrians from all traffic on Alma and Churchill.
The other end of the tunnel would connect with the bike trail next to Palo Alto High School. This is a convenient place for the tunnel since the connection to the bike trail is very simple and the tunnel is still close to the High School. It also doesn't require an encroachment on the Caltrain right of way for access ramps since the bike trail already exists.
A view of the west end of the tunnel (on the bike trail) is shown below. This is looking south toward Embarcadero Ave. The tunnel entrance on the right connects to Kellogg Ave.
Underpass Construction Techniques
It is hoped that some clever techniques for constructing rail underpasses can be applied at Churchill. These involve constructing the rail bridge around or next to the tracks without disrupting train operations, then interrupting rail service for only a long weekend to slide the bridge structure into place. If practical, this will eliminate the need for shooflyA shoofly is a temporary detour around a construction zone or other blockage on the mainline tracks. tracks, shortening the schedule, lowering the amount of disruption, and reducing the cost.
Some examples of such techniques:
Art Engineering has a technique for constructing rail bridges starting with pilings on either side of the tracks, then sliding a bridge structure underneath the tracks in one short operation. Some interesting videos (sales materials) can be found at: Art Engineering, Inc.
Another technique is to construct a pre-fabricated box next to the tracks, pause rail operations, then shove the box underneath the tracks. This has been used on the Long Island Railroad to construct underpasses more complex than what will be needed at Churchill. You see a summary and some video at: Long Island Railroad.
Another, more relevant example, is the construction of a pedestrian tunnel under Caltrain in Santa Clara. This required no shooflyA shoofly is a temporary detour around a construction zone or other blockage on the mainline tracks. and interrupted Caltrain for only a Thanksgiving weekend. You can see a timelapse of the construction at Santa Clara Pedestrian Tunnel,