# Washington State Route 18
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{{Short description|State highway in King County, Washington, US}}
{{Use American English|date=May 2025}}
{{Use mdy dates|date=January 2025}}
{{Infobox road
| state = WA
| type = SR
| route = 18
| section = 075
| map = {{maplink|frame=yes|plain=yes|frame-align=center|frame-width=290|frame-height=290|frame-lat=47.399|frame-long=-122.100|zoom=9|type=line|raw={{Wikipedia:Map data/Wikipedia KML/Washington State Route 18}}}}
| map_custom = yes
| map_notes = SR 18 highlighted in red
| length_mi = 28.41
| length_round = 2
| length_ref = [{{cite web |author=Staff |year=2012 |title=State Highway Log: Planning Report 2012, SR 2 to SR 971 |publisher=[Washington State Department of Transportation](/source/Washington_State_Department_of_Transportation) |url=http://www.wsdot.wa.gov/mapsdata/roadway/pdf/HwyLog2012Statewide.pdf |pages=505–515 |access-date=April 4, 2013}}]
| established = 1964[{{cite web |url=http://apps.leg.wa.gov/RCW/default.aspx?cite=47.17.075 |title=47.17.075: State route No. 18 |year=1970–1987|work=[Revised Code of Washington](/source/Revised_Code_of_Washington) |publisher=[Washington State Legislature](/source/Washington_State_Legislature) |access-date=April 4, 2013}}]
| direction_a = West
| terminus_a = {{jct|state=WA|SR|99}} in [Federal Way](/source/Federal_Way%2C_Washington)
| junction = {{jct|state=WA|SR|161}} in Federal Way
{{jct|state=WA|I|5}} in Federal Way
{{jct|state=WA|SR|167}} in [Auburn](/source/Auburn%2C_Washington)
{{jct|state=WA|SR|164}} in Auburn
{{jct|state=WA|SR|516}} in [Covington](/source/Covington%2C_Washington)
{{jct|state=WA|SR|169}} in [Maple Valley](/source/Maple_Valley%2C_Washington)
| direction_b = East
| terminus_b = {{jct|state=WA|I|90}} near [Snoqualmie](/source/Snoqualmie%2C_Washington)
| county = King
| previous_type = SR
| previous_route = 17
| next_type = SR
| next_route = 19
}}
'''State Route 18''' ('''SR 18''') is a {{convert|28.41|mi|km|adj=mid|-long}} [state highway](/source/State_highways_in_Washington) in the [U.S. state](/source/U.S._state) of [Washington](/source/Washington_(state)), serving southeastern [King County](/source/King_County%2C_Washington). The highway travels northeast, primarily as a [controlled-access freeway](/source/Controlled-access_highway), from an intersection with [SR 99](/source/Washington_State_Route_99) and an interchange with [Interstate 5](/source/Interstate_5_in_Washington) (I-5) in [Federal Way](/source/Federal_Way%2C_Washington) through the cities of [Auburn](/source/Auburn%2C_Washington), [Kent](/source/Kent%2C_Washington), [Covington](/source/Covington%2C_Washington), and [Maple Valley](/source/Maple_Valley%2C_Washington). SR 18 becomes a two-lane rural highway near [Tiger Mountain](/source/Tiger_Mountain_(Washington)) as it approaches its eastern terminus, an interchange with [I-90](/source/Interstate_90_in_Washington) near the cities of [Snoqualmie](/source/Snoqualmie%2C_Washington) and [North Bend](/source/North_Bend%2C_Washington).
SR 18 was established during the [1964 state highway renumbering](/source/1964_state_highway_renumbering_(Washington)) as the successor to the Auburn–Federal Way branch of [Primary State Highway 5](/source/Primary_State_Highway_5_(Washington)) (PSH 5) and the Auburn–North Bend branch of [PSH 2](/source/Primary_State_Highway_2_(Washington)), which were created in 1931 and 1949, respectively. The initial two-lane highway, named the '''Echo Lake Cutoff''', was completed in December 1964 after the opening of a section around Tiger Mountain, which would later be the site of over 170 accidents in the 1980s. SR 18 was gradually widened into a four-lane freeway beginning in Auburn in 1992 and most recently finishing in Federal Way in 2007. The highway around Tiger Mountain and near the I-90 interchange remains a two-lane road, with a funded project planned to re-build the existing interchange with I-90.
==Route description==
[[File:I-5 south at WA-18, Federal Way.jpg|thumb|left|[I-5](/source/Interstate_5_in_Washington) southbound in [Federal Way](/source/Federal_Way%2C_Washington), approaching its interchange with SR 18]]
SR 18 begins as South 348th Street at a [signalized intersection](/source/Traffic_light) with [SR 99](/source/Washington_State_Route_99), named the [Pacific Highway](/source/Pacific_Highway_(United_States)), in the city of [Federal Way](/source/Federal_Way%2C_Washington). The highway travels due east through an intersection with the Enchanted Parkway, which carries [SR 161](/source/Washington_State_Route_161) southwards towards [Wild Waves Theme Park](/source/Wild_Waves_Theme_Park) and the city of [Puyallup](/source/Puyallup%2C_Washington), to a hybrid [cloverleaf](/source/Cloverleaf_interchange)-[stack](/source/Stack_interchange) interchange with [I-5](/source/Interstate_5_in_Washington), providing access to [Seattle](/source/Seattle%2C_Washington) and [Vancouver, British Columbia](/source/Vancouver) to the north, and [Tacoma](/source/Tacoma%2C_Washington) and [Portland, Oregon](/source/Portland%2C_Oregon) to the south.[{{cite web |date=April 14, 2009 |title=SR 5 - Exit 142: Junction SR 18 |publisher=Washington State Department of Transportation |url=http://www.wsdot.wa.gov/mapsdata/tools/InterchangeViewer/pdf/SR005/005X142.pdf |access-date=April 14, 2013}}] SR 18 becomes a full four-lane freeway as it descends into [Peasley Canyon](/source/Peasley_Canyon) east of a [diamond interchange](/source/diamond_interchange) with Weyerhaeuser Way,[{{cite web |date=November 16, 2010 |title=SR 18: Junction Weyerhaeuser Way S |publisher=Washington State Department of Transportation |url=http://www.wsdot.wa.gov/mapsdata/tools/InterchangeViewer/pdf/SR018/018X000.pdf |access-date=April 14, 2013}}] located south of the [Weyerhaeuser](/source/Weyerhaeuser) headquarters.[{{cite web |title=Corporate Headquarters Region: Driving Directions |url=http://www.weyerhaeuser.com/Company/CorporateAffairs/Directions |publisher=[Weyerhaeuser](/source/Weyerhaeuser) |access-date=April 14, 2013 |archive-date=May 18, 2013 |archive-url=https://web.archive.org/web/20130518072713/http://www.weyerhaeuser.com/Company/CorporateAffairs/Directions |url-status=dead }}][{{google maps |title=State Route 18 |url=https://maps.google.com/maps?saddr=S+348th+St&daddr=WA-18+E&hl=en&ll=47.39925,-122.10073&spn=0.248656,0.676346&sll=47.508795,-121.882786&sspn=0.001939,0.005284&geocode=FamV0QId5JO1-A%3BFQfu1AIdIDS8-A&mra=me&mrsp=1,0&sz=18&t=m&z=11 |access-date=July 19, 2009}}][{{cite map |title=Washington State Highways, 2011–2012 |scale=1:842,000 |year=2011 |url=http://www.wsdot.wa.gov/NR/rdonlyres/14A6187A-B266-4340-A351-D668F89AC231/0/TouristMapFront_withHillshade.pdf |format=PDF |publisher=Washington State Department of Transportation |access-date=April 14, 2013 |archive-date=July 26, 2012 |archive-url=https://web.archive.org/web/20120726042334/http://www.wsdot.wa.gov/NR/rdonlyres/14A6187A-B266-4340-A351-D668F89AC231/0/TouristMapFront_withHillshade.pdf |url-status=dead }}]
After exiting the canyon, the freeway enters the city of [Auburn](/source/Auburn%2C_Washington) and intersects West Valley Highway, signed as [SR 181](/source/Washington_State_Route_181) until 1991, and [SR 167](/source/Washington_State_Route_167), named the Valley Freeway, in a complex hybrid [partial cloverleaf](/source/partial_cloverleaf_interchange) and diamond interchange.[{{cite web |date=March 29, 2012 |title=SR 18: Junction SR 167/W Valley Highway |publisher=Washington State Department of Transportation |url=http://www.wsdot.wa.gov/mapsdata/tools/InterchangeViewer/pdf/SR018/018X002.pdf |access-date=April 14, 2013}}] SR 18 continues east past [The Outlet Collection Seattle](/source/The_Outlet_Collection_Seattle) and over the mixed-use [Interurban Trail](/source/Interurban_Trail_(King_County)) as it approaches a [folded cloverleaf interchange](/source/Partial_cloverleaf_interchange) with C Street Southwest and a partial cloverleaf interchange with [SR 164](/source/Washington_State_Route_164) at Auburn Way.[{{cite map |publisher=[King County](/source/King_County%2C_Washington) |title=King County Bicycling Guidemap |date=March 2010 |url=http://your.kingcounty.gov/kcdot/roads/wcms/bike/BikeMapSouth2010.pdf |format=PDF |access-date=April 14, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20120305024455/http://your.kingcounty.gov/kcdot/roads/wcms/bike/BikeMapSouth2010.pdf |archive-date=March 5, 2012 }}][{{cite web |date=January 27, 2006 |title=SR 18: Junction C Street SW |publisher=Washington State Department of Transportation |url=http://www.wsdot.wa.gov/mapsdata/tools/InterchangeViewer/pdf/SR018/018X003.pdf |access-date=April 14, 2013}}][{{cite web |date=March 2, 2001 |title=SR 18: Junction SR 164/Auburn Way |publisher=Washington State Department of Transportation |url=http://www.wsdot.wa.gov/mapsdata/tools/InterchangeViewer/pdf/SR018/018X003.pdf |access-date=April 14, 2013}}] The freeway then crosses over the [Green River](/source/Green_River_(Duwamish_River)) into [unincorporated](/source/Unincorporated_area) [King County](/source/King_County%2C_Washington).[{{cite web |date=September 21, 2004 |title=SR 18: Junction Auburn-Black Diamond Road |publisher=Washington State Department of Transportation |url=http://www.wsdot.wa.gov/mapsdata/tools/InterchangeViewer/pdf/SR018/018X006.pdf |access-date=April 14, 2013}}]
SR 18 continues northeast along the southeastern [city limit](/source/city_limit)s of [Kent](/source/Kent%2C_Washington), through an interchange with Southeast 304th Street, which serves [Green River College](/source/Green_River_College),[{{cite web |date=September 21, 2004 |title=SR 18: Junction SE 304th Street |publisher=Washington State Department of Transportation |url=http://www.wsdot.wa.gov/mapsdata/tools/InterchangeViewer/pdf/SR018/018X008.pdf |access-date=April 14, 2013}}] towards [Covington](/source/Covington%2C_Washington). The freeway intersects [SR 516](/source/Washington_State_Route_516) in a diamond interchange and 256th Street Southeast in a partial cloverleaf interchange before leaving Covington.[{{cite web |date=September 21, 2004 |title=SR 18: Junction SR 516 |publisher=Washington State Department of Transportation |url=http://www.wsdot.wa.gov/mapsdata/tools/InterchangeViewer/pdf/SR018/018X011.pdf |access-date=April 14, 2013}}][{{cite web |date=September 21, 2004 |title=SR 18: Junction SE 256th Street |publisher=Washington State Department of Transportation |url=http://www.wsdot.wa.gov/mapsdata/tools/InterchangeViewer/pdf/SR018/018X013.pdf |access-date=April 14, 2013}}] SR 18 intersects Southeast 231st Street in a diamond interchange located north of [Maple Valley](/source/Maple_Valley%2C_Washington), providing a connection to [SR 169](/source/Washington_State_Route_169), while the freeway travels on an overpass over SR 169 towards a partial cloverleaf interchange with 244th Avenue.[{{cite web |date=December 21, 2011 |title=SR 18: Junction SR 169/SE 231st Street |publisher=Washington State Department of Transportation |url=http://www.wsdot.wa.gov/mapsdata/tools/InterchangeViewer/pdf/SR018/018X015.pdf |access-date=April 14, 2013}}][{{cite web |date=April 25, 2012 |title=SR 18: Junction 244th Avenue |publisher=Washington State Department of Transportation |url=http://www.wsdot.wa.gov/mapsdata/tools/InterchangeViewer/pdf/SR018/018X018.pdf |access-date=April 14, 2013}}] The SR 18 freeway ends northeast of a partial cloverleaf interchange with Issaquah-Hobart Road at the base of [Tiger Mountain](/source/Tiger_Mountain_(Washington)),[{{cite web |date=March 23, 2007 |title=SR 18: Junction Issaquah-Hobart Road/276th Avenue |publisher=Washington State Department of Transportation |url=http://www.wsdot.wa.gov/mapsdata/tools/InterchangeViewer/pdf/SR018/018X020.pdf |access-date=April 14, 2013}}] becoming a two-lane highway with at-grade intersections for the remainder of its route. The highway travels on the east side of Tiger Mountain in the [Issaquah Alps](/source/Issaquah_Alps) and provides access to a [trailhead](/source/trailhead) in [Tiger Mountain State Forest](/source/Tiger_Mountain_(Washington)) before it reaches its eastern terminus, a diamond interchange with [I-90](/source/Interstate_90_in_Washington) located west of [North Bend](/source/North_Bend%2C_Washington).[{{cite web |date=October 25, 2011 |title=SR 90 - Exit : Junction SR 18/Echo Glen Road |publisher=Washington State Department of Transportation |url=http://www.wsdot.wa.gov/mapsdata/tools/InterchangeViewer/pdf/SR090/090X025.pdf |access-date=April 14, 2013}}] I-90 provides access to the cities of [Issaquah](/source/Issaquah%2C_Washington) and [Seattle](/source/Seattle) to the west and North Bend and [Spokane](/source/Spokane%2C_Washington) to the east, traveling over the [Cascade Mountains](/source/Cascade_Mountains) through [Snoqualmie Pass](/source/Snoqualmie_Pass). The roadway continues past the interchange as Snoqualmie Parkway into the city of [Snoqualmie](/source/Snoqualmie%2C_Washington), intersecting [SR 202](/source/Washington_State_Route_202).[{{cite map |title=King County Highways and Incorporated Areas |date=January 2013 |url=http://your.kingcounty.gov/ftp/gis/Web/VMC/misc/KC_HwysCitiesHS.pdf |format=PDF |publisher=King County GIS Center |access-date=April 14, 2013}}]
Every year, the [Washington State Department of Transportation](/source/Washington_State_Department_of_Transportation) (WSDOT) conducts a series of surveys on its highways in the state to measure traffic volume. This is expressed in terms of [annual average daily traffic](/source/annual_average_daily_traffic) (AADT), which is a measure of traffic volume for any average day of the year. In 2012, WSDOT calculated that the busiest section of SR 18 was its interchange with SR 167 in Auburn, serving 97,000 vehicles, while the least busiest section of the highway was its eastern terminus at I-90, serving 19,000 vehicles.[{{cite web |author=Staff |year=2012 |title=2012 Annual Traffic Report |publisher=Washington State Department of Transportation |url=http://www.wsdot.wa.gov/mapsdata/travel/pdf/Annual_Traffic_Report_2012.pdf |pages=96–97 |access-date=April 14, 2013 |archive-date=December 24, 2013 |archive-url=https://web.archive.org/web/20131224111057/http://www.wsdot.wa.gov/mapsdata/travel/pdf/Annual_Traffic_Report_2012.pdf |url-status=dead }}] SR 18 is designated as part of the [National Highway System](/source/National_Highway_System_(United_States)) for its whole length,[{{cite map |publisher=[Federal Highway Administration](/source/Federal_Highway_Administration) |title=National Highway System: Seattle, WA |url=https://www.fhwa.dot.gov/planning/national_highway_system/nhs_maps/washington/seattle_wa.pdf |date=October 1, 2012 |access-date=April 14, 2013}}] classifying it as important to the national economy, defense, and mobility.[{{cite web |title=What is the National Highway System? |url=https://www.fhwa.dot.gov/planning/national_highway_system/ |publisher=Federal Highway Administration |date=September 26, 2012 |access-date=April 14, 2013}}] WSDOT designates the entire route of SR 18 as a Highway of Statewide Significance,[{{cite web |title=Transportation Commission List of Highways of Statewide Significance |url=https://www.wsdot.wa.gov/sites/default/files/2006/03/16/HSSlist2009mod2.pdf |publisher=[Washington State Transportation Commission](/source/Washington_State_Transportation_Commission) |date=July 26, 2009 |access-date=April 14, 2013 |archive-date=July 24, 2013 |archive-url=https://web.archive.org/web/20130724190950/http://www.wsdot.wa.gov/NR/rdonlyres/50EC9EB9-DB3D-4823-B5D2-5348409FB8CE/0/HSSlist2009mod2.pdf |url-status=dead }}] which includes highways that connect major communities in the [state of Washington](/source/Washington_(state)).[{{cite web |last=Lorenzo |first=Judy |title=Highways of Statewide Significance |url=http://www.wsdot.wa.gov/planning/HSS/Default.htm |publisher=Washington State Department of Transportation |access-date=April 14, 2013}}]
==History==
[[File:I-90 east at WA-18, near Snoqualmie.jpg|thumb|right|[I-90](/source/Interstate_90_in_Washington) eastbound approaching its interchange with SR 18 near Snoqualmie]]
The modern corridor that SR 18 follows was added to the state highway system in 1931 as a branch of [State Road 5](/source/State_Road_5_(Washington)) that ran from the main highway in [Auburn](/source/Auburn%2C_Washington) to [State Road 1](/source/State_Road_1_(Washington_1923-1937)) and [U.S. Route 99](/source/U.S._Route_99) (US 99) in [Federal Way](/source/Federal_Way%2C_Washington) via [Peasley Canyon](/source/Peasley_Canyon).[{{cite book |author=Washington State Legislature |title=Session Laws of the State of Washington |edition=1931 |year=1931 |publisher=Washington State Legislature |location=[Olympia, Washington](/source/Olympia%2C_Washington) |chapter=Chapter 29}}][{{cite map |publisher=[Department of Highways](/source/Department_of_Highways_(Washington)) |title=Highway Map: State of Washington |date=January 1931 |url=http://www.sos.wa.gov/history/maps_detail.aspx?m=34 |format=[DJVU](/source/DJVU) |access-date=April 14, 2013}}] The branch was retained by State Road 5 when it was re-designated as [PSH 5](/source/Primary_State_Highway_5_(Washington)) as a [new highway code](/source/Primary_State_Highways_(Washington)) was established in 1937.[{{cite book |author=Washington State Legislature |title=Session Laws of the State of Washington |edition=1937 |date=March 17, 1937 |access-date=April 14, 2013 |url=https://books.google.com/books?id=dcQ4AAAAIAAJ |publisher=Washington State Legislature |location=Olympia, Washington |chapter=Chapter 190: Establishment of Primary State Highways |pages=935–936 |quote=A primary state highway to be known as Primary State Highway No. 5, or the National Park Highway, is hereby established according to description as follows: Also beginning at Auburn on Primary State Highway No. 5, as herein described, thence in a westerly direction by the most feasible route to a junction with Primary State Highway No. 1.}}][{{cite map |publisher=Department of Highways |title=Highways of the State of Washington |year=1939 |url=http://www.sos.wa.gov/history/maps_detail.aspx?m=28 |format=DJVU |access-date=April 14, 2013}}] A branch of [PSH 2](/source/Primary_State_Highway_2_(Washington)) was designated in 1949, traveling southwest from [North Bend](/source/North_Bend%2C_Washington), around [Tiger Mountain](/source/Tiger_Mountain_(Washington)) and through [Auburn](/source/Auburn%2C_Washington) before ending at a junction with PSH 1 and US 99 in [Milton](/source/Milton%2C_Washington).[{{cite book |author=Washington State Legislature |title=Session Laws of the State of Washington |edition=1949 |date=March 22, 1949 |access-date=April 14, 2013 |url=https://books.google.com/books?id=zcc4AAAAIAAJ |publisher=Washington State Legislature |location=Olympia, Washington |chapter=Chapter 225: Establishing of Primary and Secondary State Highways |page=773 |quote=A primary state highway to be known as Primary State Highway No. 2, or the Sunset Highway, is hereby established according to description as follows: Also from a junction at a point approximately four miles west of North Bend in a general southwesterly direction by the most direct and feasible route by way of Auburn to a junction with State Road No. 1 in the vicinity of Milton.}}] The designation had been the result of lobbying from business leaders in Tacoma, who sought a direct connection to Snoqualmie Pass that would improve access to Eastern Washington.[{{cite news |last=Stansfield |first=Dick |date=September 20, 1959 |title=Road Crews Pushing Echo Lake Cutoff Toward North Bend to Link Tacoma to Snoqualmie Pass |page=A12 |work=[The News Tribune](/source/The_News_Tribune) |url=https://www.newspapers.com/clip/98534873/road-crews-pushing-echo-lake-cutoff/ |via=[Newspapers.com](/source/Newspapers.com) |accessdate=April 7, 2022}}]
Construction on the Echo Lake Cutoff Road, along the route of the North Bend–Auburn branch of PSH 2 and the Auburn–Federal Way branch PSH 5, began in 1955. The two-lane highway would cost $9 million (equivalent to ${{formatnum:{{Inflation|US|9|1955|r=0}}}} million in {{inflation/year|US}}{{Inflation-fn|US}}) and the {{convert|32|mi|km|adj=mid|-long}} route was chosen to avoid Tiger Mountain and connect the city of [Tacoma](/source/Tacoma%2C_Washington) to [Snoqualmie Pass](/source/Snoqualmie_Pass).[{{cite news |title=Echo Lake Cutoff Now Completed |date=November 26, 1964 |page=3 |url=https://news.google.com/newspapers?id=lNxXAAAAIBAJ&sjid=5OgDAAAAIBAJ&pg=6761,3139885&dq=echo+lake+cutoff&hl=en |work=[The Spokesman-Review](/source/The_Spokesman-Review) |access-date=April 14, 2013}}] The first section, between US 99 and Auburn, was opened on December 19, 1958.[{{cite news |date=December 18, 1958 |title=Highway Cutoff Opens Tomorrow |page=10 |work=[The Seattle Times](/source/The_Seattle_Times) |agency=[Associated Press](/source/Associated_Press)}}][{{cite news |date=February 1959 |title=First Section of Echo Lake Cutoff Opened to Auburn |page=7 |work=Washington Highway News |publisher=Washington State Department of Highways |volume=8 |issue=4 |oclc=29654162 |url=https://cdm16977.contentdm.oclc.org/digital/collection/p16977coll1/id/3390/rec/5 |via=Washington State Department of Transportation Library Digital Collections |access-date=August 29, 2018}}] SR 18 was designated on the corridor during the [1964 state highway renumbering](/source/1964_state_highway_renumbering_(Washington)) and codified into state law in 1970, replacing the branches of PSH 2 and PSH 5.[{{cite web |last=Prahl |first=C. G. |title=Identification of State Highways |url=https://www.wsdot.wa.gov/sites/default/files/2005/04/26/Identification-of-state-highways.pdf |publisher=Washington State Highway Commission, Department of Highways |date=December 1, 1965 |access-date=April 14, 2013 |archive-date=February 2, 2020 |archive-url=https://web.archive.org/web/20200202073838/https://www.wsdot.wa.gov/sites/default/files/2005/04/26/Identification-of-state-highways.pdf |url-status=dead }}] The final {{convert|7|mi|km|2}} of the Echo Lake Cutoff, from an entrance to [Tiger Mountain State Forest](/source/Tiger_Mountain_(Washington)) to [I-90](/source/Interstate_90_in_Washington), was officially opened on December 1, 1964.[{{cite news |title=Echo Lake Road Link Opened |date=December 2, 1964 |page=6 |url=https://news.google.com/newspapers?id=-1pYAAAAIBAJ&sjid=lfcDAAAAIBAJ&pg=5402,454557&dq=echo+lake+cutoff&hl=en |work=[Spokane Daily Chronicle](/source/Spokane_Daily_Chronicle) |access-date=April 14, 2013}}] A new interchange with [I-5](/source/Interstate_5_in_Washington) in Federal Way was opened to traffic on January 31, 1967,[{{cite news |last=Duncan |first=Don |title=The Way We Were |date=August 13, 1985 |page=C4 |work=The Seattle Times}}] officially completing SR 18.[{{cite web |last=Wilma |first=David |date=September 4, 2002 |title=Interstate 5 is completed from Everett to Tacoma on January 31, 1967. |url=http://www.historylink.org/index.cfm?displaypage=output.cfm&file_id=1354 |work=HistoryLink |access-date=April 14, 2013}}]
Expansion of SR 18 from a two-lane rural road to a four-lane [controlled-access freeway](/source/Controlled-access_highway) began in 1992 response to six fatalities in over 170 accidents in a ten-year period, giving the highway a reputation of being a "dangerous roadway".[{{cite news |last=Aweeka |first=Charles |title=Big plans to tame deadly roadway—Fall work slated for two-mile leg of Highway 18 |date=June 28, 1993 |url=https://archive.seattletimes.com/archive/19930628/1708662/big-plans-to-tame-deadly-roadway----fall-work-slated-for-two-mile-leg-of-highway-18 |work=The Seattle Times |access-date=July 19, 2009}}][{{cite news |last=Watts |first=Lyn |date=March 27, 1992 |title=$1.16 million project aims to strip 'killer road' tag |page=B1 |work=[The News Tribune](/source/The_News_Tribune) |url=https://www.newspapers.com/clip/85890861/116-million-project-aims-to-strip-kil/ |via=[Newspapers.com](/source/Newspapers.com) |access-date=September 24, 2021}}] [WSDOT](/source/Washington_State_Department_of_Transportation) widened SR 18 to four lanes and added new interchanges between [SR 167](/source/Washington_State_Route_167) and the [Green River](/source/Green_River_(Duwamish_River)) within Auburn.[{{cite news |date=August 2, 1995 |title=Construction will slow travel on Highway 18 |page=B2 |work=The News Tribune |url=https://www.newspapers.com/clip/85890871/construction-will-slow-travel-on/ |via=Newspapers.com |access-date=September 24, 2021}}] The [Washington State Legislature](/source/Washington_State_Legislature) briefly planned to toll the new SR 18 freeway in January 1995 to pay off loans for re-construction, but the plan was protested by local residents and rejected months later in May.[{{cite news |last1=Turner |first1=Joseph |last2=Kremer |first2=Lisa |date=January 27, 1995 |title=Blistered officials dump plan to levy tolls on Washington 18 |page=A1 |work=The News Tribune |url=https://www.newspapers.com/clip/85890887/blistered-officials-dump-plan-to-levy/ |via=Newspapers.com |access-date=September 24, 2021}}][{{cite news |last=Turner |first=Joseph |date=May 17, 1995 |title=Narrows Bridge may be sole survivor of toll-road plan |page=A1 |work=The News Tribune |url=https://www.newspapers.com/clip/85890907/narrows-bridge-may-be-sole-survivor-of/ |via=Newspapers.com |access-date=September 24, 2021}}] Additional work was completed in 1997 from the Green River to Southeast 304th Street in [Covington](/source/Covington%2C_Washington),[{{cite news |last=Epler |first=Patti |date=April 10, 1996 |title=Construction in progress on Washington 18 |page=B2 |work=The News Tribune |url=https://www.newspapers.com/clip/85890895/construction-in-progress-on-washington/ |via=Newspapers.com |access-date=September 24, 2021}}] and in 2007 from Covington through [Maple Valley](/source/Maple_Valley%2C_Washington) and to Issaquah-Hobart Road.[{{cite news |last=Lange |first=Larry |title=Getting There: Will Route 18 widening work never end? |date=April 3, 2005 |url=http://www.seattlepi.com/local/transportation/article/Getting-There-Will-Route-18-widening-work-never-1170079.php |work=[Seattle Post-Intelligencer](/source/Seattle_Post-Intelligencer) |access-date=April 14, 2013}}][{{cite web |last=Phipps |first=Greg |title=SR 18 - Maple Valley to Issaquah Hobart Road - Complete July 2007 |date=July 2007 |url=http://www.wsdot.wa.gov/Projects/SR18/AuburntoI90/MValley_IHobart/ |publisher=Washington State Department of Transportation |access-date=April 14, 2013}}] The western end of the freeway was extended in Federal Way through Peasley Canyon to I-5 in Federal Way,[{{cite news |last=Brown |first=Charles |title=Bumper to Bumper: Why no warning before 520 bridge? |date=October 16, 2005 |url=http://seattletimes.com/html/localnews/2002564081_bumper16m.html |work=The Seattle Times |access-date=April 14, 2013}}] which was further improved in 2011 with the completion of interchange improvements to the "Federal Way Triangle".[{{cite news |last=Glenn |first=Stacia |title=Final ramp opens today as part of Federal Way Triangle project |date=September 26, 2012 |url=http://blog.thenewstribune.com/traffic/2012/09/26/final-ramp-opens-today-as-part-of-federal-way-triangle-project/ |work=The News Tribune |access-date=April 14, 2013 |archive-date=September 30, 2012 |archive-url=https://web.archive.org/web/20120930054939/http://blog.thenewstribune.com/traffic/2012/09/26/final-ramp-opens-today-as-part-of-federal-way-triangle-project/ |url-status=dead }}][{{cite web |last=Borschowa |first=Aleta |title=I-5 - SR 161/SR 18 Triangle Improvements |url=http://www.wsdot.wa.gov/projects/i5/sr18sr161ic/ |publisher=Washington State Department of Transportation |access-date=April 14, 2013 |archive-date=January 24, 2013 |archive-url=https://web.archive.org/web/20130124163656/http://www.wsdot.wa.gov/projects/i5/sr18sr161ic |url-status=dead }}]
WSDOT is planning to widen the remaining section of SR 18, between Issaquah-Hobart Road south of Tiger Mountain to I-90 near Snoqualmie, to four lanes with a [median barrier](/source/jersey_barrier) to separate opposing lanes of traffic and prevent head-on collisions. The western section will include a [right-in/right-out](/source/right-in%2Fright-out) interchange at the Tiger Mountain summit; $640 million was approved in the 2022 Move Ahead Washington transportation package.[{{cite news |last=Wilson |first=Conor |date=February 18, 2022 |title=Funds for Snoqualmie Parkway, SR 18 included in state transit proposal |url=https://www.issaquahreporter.com/news/funds-for-snoqualmie-parkway-sr-18-included-in-state-transit-proposal/ |work=Issaquah Reporter |accessdate=March 14, 2022}}][{{cite web |title=SR 18 - Issaquah/Hobart Rd to Deep Creek - Widening |url=https://wsdot.wa.gov/construction-planning/search-projects/sr-18-issaquah-hobart-rd-deep-creek-widening |publisher=Washington State Department of Transportation |accessdate=March 14, 2022}}] The I-90 interchange is planned to be rebuilt as part of a separate project scheduled to be completed in 2025;[{{cite press release |date=January 13, 2022 |title=Contractor selection marks milestone for I-90/SR 18 interchange improvement project near North Bend |url=https://wsdot.wa.gov/about/news/2022/contractor-selection-marks-milestone-i-90sr-18-interchange-improvement-project-near-north-bend |publisher=Washington State Department of Transportation |accessdate=March 14, 2022}}] it was originally set to include [flyover ramp](/source/flyover_ramp)s and a [double roundabout](/source/double_roundabout_interchange), but was later changed to a [diverging diamond interchange](/source/diverging_diamond_interchange).[{{cite news |last=Kiggins |first=Steve |date=June 16, 2017 |title='Diverging diamond' may ease congestion plaguing I-90 and SR 18 interchange |url=http://q13fox.com/2017/06/16/diverging-diamond-possible-for-congestion-plagued-sr-18-i-90-interchange/ |publisher=[Q13 Fox News](/source/KCPQ-TV) |access-date=January 12, 2018}}][{{cite web |title=SR 18 - Issaquah Hobart Road to I-90 - Preferred Alternative |url=http://www.wsdot.wa.gov/Projects/SR18/AuburntoI90/IHobart_I90/alternative.htm |publisher=Washington State Department of Transportation |url-status=dead |archive-url=https://web.archive.org/web/20130204114857/http://www.wsdot.wa.gov/Projects/SR18/AuburntoI90/IHobart_I90/alternative.htm |archive-date=February 4, 2013 |access-date=April 14, 2013 }}]
==Major intersections==
{{WAinttop|county=King|length_ref=