The Park and the Automobile
The greatest urban impact on Rock Creek Park and the Rock Creek and Potomac Parkway was and is automobile traffic. Although virtually all national parklands felt the heavy influence of the automobile, few were so dominated by it.
Automobiles could not be accused of intruding in the Rock Creek and Potomac Parkway, which was intended from its inception to accommodate them. "The road in the Rock Creek and Potomac Parkway section is designed for high speed traffic as far north as Cathedral Avenue," reported Thomas C. Jeffers, a National Capital Park and Planning Commission landscape architect, in 1934. "Its real purpose is to provide a pleasant and speedy way of travel between Potomac Park and Rock Creek Park." Jeffers contrasted it with Beach Drive and the link through the zoo, which he declared must remain low speed and unstraightened. 
Upon completion of the parkway road in June 1936, Chairman Frederic A. Delano of the Park and Planning Commission suggested alternating one-way traffic south and north for morning and evening commuters respectively. This pattern was inaugurated in May 1937 and became a permanent feature of the parkway, reinforcing its status as a commuter route. Among the regular parkway commuters during the first decade was Secretary Ickes, who in the early 1940s requested weekly park police reports on violators of the one-way traffic regulation and personally recorded the license numbers of offenders he spotted.
The completion and heavy use of the parkway road created new pressures on the valley road to the north. The link from the upper end of the parkway through the zoo to Rock Creek Park proper, built in the 1920s, was a major impediment to through traffic. It wound sharply along an S-curve of the creek and traversed two fords, which caused closure of the road during high water.
In 1933, even before the parkway road was finished, highway improvement advocates in the city were favoring construction of a road tunnel beneath a portion of the zoo to straighten the link. The Smithsonian Institution, which administered the zoo and the road segment through it, opposed the tunnel. So did the National Capital Park and Planning Commission, which thought it would violate the park character of the road. But the highway interests, generally supported by the District of Columbia Board of Commissioners, persisted and broadened their vision. In 1938 Commissioner Melvin C. Hazen proposed extension of a double traffic artery through the zoo and north through the Rock Creek valley to East-West Highway in Maryland--a scheme opposed by the Evening Star newspaper as "about the worst thing that could happen to Rock Creek Park." 
Prompted by the District commissioners, Congress in the fiscal 1940 District appropriations act ordered planning for "additional highway and parkway facilities in the vicinity of, into and through Rock Creek Park, Rock Creek and Potomac connecting Parkway and National Zoological Park." But the National Capital Park and Planning Commission and the National Park Service, essential parties to the planning, succeeded installing action until 1942, when World War II shifted federal priorities and the through highway scheme was shelved. 
The zoo road became a live issue again in 1954, with the National Park Service and National Capital Planning Commission (as it was retitled in 1952) now eager to improve that segment. The Service advocated twin two-lane tunnels through the hill around which the downstream bend of the creek flowed through the zoo, with a bridge over the creek below the south portals. The Smithsonian was still reluctant and deflected Service requests for permission to start surveying and test boring for the tunnels. "This matter has been very carefully discussed in our offices, and I am afraid that it is the unanimous opinion here that it would be disadvantageous for the National Zoological Park to have the road you describe cut through its property...," Secretary Leonard Carmichael wrote Director Conrad L. Wirth in February 1957. "It is our considered opinion that an arterial type road cutting through this property would seriously interfere with the basic recreational and scientific functions of the Zoological Park." 
In a peremptory reply to Carmichael, Wirth spelled out the legislative intent of Rock Creek and Potomac Parkway as connecting Potomac Park and Rock Creek Park and virtually demanded a transfer of zoo land to the Service for the purpose. Unintimidated, Carmichael replied that much more time would be needed for the Smithsonian's lawyers and board of regents to consider the matter. The Service then took a more conciliatory approach, wooing Dr. Theodore H. Reed, the zoo director, with master planning assistance. In a 1959 letter to Carmichael, Wirth was deferential, promising to hold the new zoo road to two lanes and offering to allow the Smithsonian to choose its alignment. Carmichael finally authorized the survey and borings, and in March 1960 he announced the Smithsonian's approval of plans prepared by the Service and Bureau of Public Roads. The two-lane road would tunnel through "Administration Hill" and follow the east side of Rock Creek to Klingle Road; the zoo land north of the tunnel and east of the road would be transferred to the Service; and the Service, would build a parking lot for the zoo near the Harvard Street entrance, and a bridge to carry Harvard Street traffic across the parkway. 
After further design work, a contract for $1,536,584 was let to A. S. Wilkerstrom, Inc., of Skaneateles, New York, in June 1962. The tunnel and new road segment, eliminating the two fords, opened to traffic in the fall of 1966.
The improvement, so long advocated by the Park Service, proved a mixed blessing. The parkway below carried more northbound rush-hour traffic wanting to use the single northbound lane of the new road through the tunnel than it could accommodate, leading to long backups on the parkway. Pressures mounted to extend one-way northbound traffic to Klingle Street--but that would only relocate the bottleneck and clog both lanes of the tunnel, blocking it to emergency vehicles and creating possible hazards from exhaust fumes.
By the 1970s the Service realized the futility of parkway improvements to lessen traffic congestion: more lanes and fewer impediments only served to attract more traffic, with increasingly evident degradation of park values. In a 1977 planning document on Rock Creek Park the Service reviewed its past actions with regret: "[T]he conversion of the Rock Creek and Potomac Parkway to exclusive use by one-way traffic during 85 morning and evening rush periods and the provision of first priority maintenance of an excellent road net directly encourage excessive commuter traffic Construction of a tunnel on roadway near the National Zoo represents inappropriate development since it directly encourages adverse increased commuter use of park roads."  Such are the insights of hind-sight!
The Service's position on the proposed four-lane arterial highway to Maryland has better stood the test of time. As advanced by Commissioner Hazen in 1938, the highway would pass through the zoo and the lower part of Rock Creek Park, extend north along the east side of Broad Branch Road to Military Road, then use Oregon Avenue widened on the park side to connect with a widened Beach Drive in Maryland. In delaying action on the proposal, the Service cited the 1918 Olmsted Report to buttress its view that no major roadway should occupy the narrow winding floor or steep wooded hillsides of the valley above the zoo (although certain crossings would be appropriate). 
The war sidelined but did not bury the highway plan, which reappeared in the Recommended Highway Improvement Program presented by the Regional Highway Planning Committee in 1952. The arterial along and through Rock Creek Park was now to be a link connecting U.S. Route 240 (present Interstate 270) in Maryland with downtown Washington. Proponents of the plan, including the District commissioners, argued that only a small part of the park--the most densely wooded and least used part--would be affected. They were joined in Maryland by the State Roads Commission and the Maryland-National Capital Park and Planning Commission, which was prepared to allow use of Maryland's Rock Creek Park for the highway and for a segment of what would become the Capital Beltway. 
The National Park Service resumed its opposition. It was aided by a group of Chevy Chase, Maryland, residents led by Gerald P. Nye, a former senator from North Dakota, who would be disturbed by a leg of the highway displacing or passing near their homes. In June 1953 Secretary of the Interior Douglas McKay wrote Nye, "[T]he Department of the Interior will vigorously oppose any use of the Rock Creek Valley for arterial highway purposes or any other use contrary to the intent of Congress in the establishment of this important park area."  Park Service Director Conrad Wirth represented Interior on the National Capital Planning Commission, which would have to approve highway construction in the Maryland parkland acquired with federal funds under the Capper-Cramton Act. The commission opposed the highway down Rock Creek Park in the District, but it overrode Wirth's objections and allowed the Capital Beltway to pass through a portion of the park in Maryland.
In January 1955 the Service drafted legislation, introduced by Chairman James E. Murray of the Senate Interior and Insular Affairs committee, that would require the National Capital Planning Commission to rescind its permission for the beltway leg, restrict its approval of subsequent roads in the Maryland park, and proscribe additional roads in the District park without specific congressional approval. The bill won support from retired congressman Louis C. Cramton, Maj. Gen. U. S. Grant III (now president of the American Planning and Civic Association), and an array of civic and conservation organizations. It was opposed by the Bureau of the Budget, which thought it unnecessary in view of existing protections; by Maryland politicians and officials, to whom it represented unwarranted r federal interference in state affairs; and by the Washington Post, which editorialized, "[N]o assurance can be given that the enormous increase in traffic from Montgomery County into the District can be handled without an expressway along the edge of Rock Creek Park some time in the future." 
The highway issue was thoroughly ventilated in a Senate hearing on the bill, which was not brought to a vote in either house then or following its reintroduction in the next Congress. Planning for the beltway leg in Maryland proceeded amid state assurances that it would be a low-speed "parkway" from which commercial traffic would be forever barred--assurances that were forgotten when the beltway was completed in the mid-1960s and became part of the interstate highway system. But opposition to bringing U.S. 240 into the District via Rock Creek Park spread and solidified. When District and Maryland highway interests revived the scheme in 1957, the Washington Post admitted past error and declared, "This would be intolerable, and Washingtonians who love their park had better rise up and block any such encroachment." (At the same time, it favored an alternate route through Glover-Archbold Park as "the least disadvantageous course," because "unquestionably a connection with U.S. 240 must be provided.") Secretary of the Interior Fred A. Seaton solicited statements from District Commissioner Robert E. McLaughlin, Secretary of Commerce Sinclair Weeks, and Gov. Theodore H. McKeldin of Maryland that they shared his strong objection to a Rock Creek highway; only McKeldin declined to join in. 
Some skirmishing continued (highway dragons being notoriously difficult to slay). A 1958 Maryland-National Capital Park and Planning Commission staff re,port on alternative extensions of U.S. 240 explicitly ruled out Rock Creek Park based on Interior-Park Service and conservationist opposition. A year later, however, the Maryland commission and the Montgomery County Planning Board resolved to restudy a route using the park. Once again Director Wirth made clear the Service position, with evident success--for a time. 
The next and last serious challenge came in 1966. The Lands Committee of the Metropolitan Washington Council of Governments and the National Capital Regional Planning Council, an affiliate of the National Capital Planning Commission, proposed a new feasibility study of a highway route along the western edge of Rock Creek Park south to Tilden Street, where it would cross to the east side and join the Rock Creek and Potomac Parkway below the zoo. The Evening Star endorsed the plan, characterizing opposition to the earlier proposals as emotional and predicting that this route would "emerge as the most reasonable, logical solution" to the need for a northwestern freeway connection. 
By this time, however, longstanding assumptions about the need for such a connection--somewhere--were being challenged by other visions. From them sprang the planning and construction of Metro, metropolitan Washington's rapid rail transit system, in the next two decades. The massive governmental commitment to Metro rendered most freeway proposals obsolete, that for Rock Creek Park among them. The park would likely have been spared without the subway, so entrenched were its defenders; but if the highway plan were finally dead, Metro entombed it.