reduce the width of both shoulders to . Existing bridges can remain part of the Interstate system if they have at least lanes with shoulder on the left and a shoulder on the right, except that longer bridges can have shoulders on both sides. For all bridges, the railing should be upgraded if necessary. Tunnel clearance: Tunnels should not differ significantly from bridges, but because of the high costs of tunnels, the width of both shoulders may be reduced to . An exit walkway wide is also required, which should either be elevated or separated from the roadway with a barrier. In addition, access for emergency responders needs to be accommodated. The minimum vertical clearance is the same as it is under bridges. Exceptions The standards have been changed over the years, resulting in many older Interstates not conforming to the current standards, and yet others are not built to standards because to do so would be too costly or environmentally unsound. Some roads were grandfathered into the system. Most of these were toll roads that were built before the Interstate system came into existence or were under construction at the time President Dwight D. Eisenhower signed the Federal Aid Highway Act of 1956. One example is the Pennsylvania Turnpike, which originally had a very narrow median that later required the installation of a Jersey barrier due to heavy traffic loads. Interstate 35E through Saint Paul, Minnesota is an example of a freeway that was not grandfathered into the system that is nonetheless an exception to standards. Initially designed in the 1960s, but not opened until 1990, the freeway has a speed limit of , and does not allow vehicles weighing over GVW. This is due to political opposition from surrounding homeowners in local neighborhoods, which greatly delayed and modified the project. Interstate 75 on the Mackinac Bridge between St. Ignace and Mackinaw City, Michigan, is undivided. The bridge was designed before the start of the Interstate Highway System, and it was grandfathered into the system. Interstate 93 through Franconia Notch, New Hampshire is also a notable exception, being a super two parkway with a speed limit of . All the unsigned Interstates in Alaska and Puerto Rico are exempt from Interstate Highway standards and are instead, per Title 23, Chapter 1, Section 103 of the U.S. Code, "designed in accordance with such geometric and construction standards as are adequate