Annapolis Road Line

Route T18 also operates alongside route T14 but under different line names and routes. Route T18 also runs a direct route while route T14 runs primarily thru neighborhoods. Route T18 operates weekly between New Carrollton station and Rhode Island Avenue–Brentwood station providing service along Annapolis Road and connecting passengers between Prince Georges County and Northeast. Route T18 also runs a limited stop segment between Mount Rainier terminal and Rhode Island Avenue-Brentwood station alongside routes G9 and T14 only stopping along five stops. Local service is provided by routes 83 and 86.

Route T18 currently operates out of Landover division.

Route T18 was created as a brand new Metrobus Route by WMATA on December 3, 1978 to operate between New Carrollton station and Rhode Island Avenue–Brentwood station shortly after New Carrollton station opened on November 20, 1978.{{Cite news |url = https://www.washingtonpost.com/archive/local/1978/11/30/bus-routes-change-sunday-to-link-with-orange-line/74b7bfbd-4100-411b-b9eb-d51f9e4a0031/ |title = Bus Routes Change Sunday to Link with Orange Line |last = Feaver |first = Douglas B. |date = November 30, 1978 |work = The Washington Post |access-date = November 3, 2019 |language = en-US |issn = 0190-8286 }}

Route T18 was designed to replace the segment of routes T14, T16, and T17 routing between Annapolis Road in New Carrollton and the Rhode Island Avenue Metro Station, via Capital Plaza Mall, since routes T14, T16, and T17 were truncated to terminate at New Carrollton.

Route T18 would operate along Ellin Road, Harkins Road, and Annapolis Road, then divert off Annapolis Road after passing the Baltimore–Washington Parkway and Capital Plaza, and make a loop along 57th Avenue, 58th Avenue, Emerson Street, and then 57th Avenue to serve Bladensburg High School, as well as the adjacent apartment complexes. Then the route returns onto Annapolis Road remaining straight on Annapolis Road and Bladensburg Road, then turn onto the intersection of 38th Avenue and 38th Street, another left turn at the intersection of Rhode Island Avenue and remain straight on Rhode Island Avenue before reaching the intersection of Washington Place NE and ultimately making a left turn onto that street to enter Rhode Island Avenue–Brentwood station. Route T18 would then turn back and operate on the same routing (except in the exact opposite direction), back towards New Carrollton station.

Beginning on December 14, 2014, route T18 began a newly limited stop segment along Rhode Island Avenue between Mount Rainier terminal and Rhode Island Avenue-Brentwood station along with route T14. The limited stop segment was to reduce bus bunching along Rhode Island Avenue and improve on time performance for route T14, and T18.{{cite web |title=Metrobus service changes effective December 14 to shorten wait times, reduce crowding |url=https://www.wmata.com/about/news/pressreleasedetail.cfm?ReleaseID=5821 |website=www.wmata.com |accessdate=21 April 2020|archive-url=https://web.archive.org/web/20190113062835/https://www.wmata.com/about/news/pressreleasedetail.cfm?ReleaseID=5821|archive-date=January 13, 2019|url-status=dead}} Buses would only serve the following stops:

• Rhode Island Ave & 12th St NE
• Rhode Island Ave & Montana Ave NE (eastbound)/14th St NE (westbound)
• Rhode Island Ave & 18th St NE
• Rhode Island Ave & South Dakota Ave NE (eastbound)/24th St NE (westbound)
• Rhode Island Ave & Newton St NE

Passengers wishing for local service will have to use routes 83 or 86.

During the COVID-19 pandemic, the T18 operated on its Saturday supplemental schedule beginning on March 16, 2020.{{cite web |title=Metro announces additional COVID-19 changes, including reduced service beginning Monday {{!}} WMATA |url=https://www.wmata.com/about/news/Metro-announces-additional-COVID-19-changes.cfm |website=www.wmata.com |access-date=31 January 2021}} It however began operating on its Sunday service on March 18, 2020.{{cite web |title=METRO SERVICE LEVELS & HOURS FURTHER REDUCED TO SUPPORT ESSENTIAL TRIPS ONLY, STARTING WEDNESDAY {{!}} WMATA |url=https://www.wmata.com/about/news/METRO-SERVICE-FURTHER-REDUCED-TO-SUPPORT-ESSENTIAL-TRIPS-ONLY.cfm |website=www.wmata.com |access-date=31 January 2021}} Weekend service was also reduced to operate every 30 munutes.{{cite web |title=Weekend: Metro service limited to 26 bus routes, reduced rail service; expect wait times of 30 minutes; customers urged to travel only if essential {{!}} WMATA |url=https://www.wmata.com/about/news/Weekend-service-3-28-3-29.cfm |website=www.wmata.com |access-date=31 January 2021}} Its regular service was restored on August 23, 2020.{{cite web |title=Metrobus Service Changes beginning August 23 {{!}} WMATA |url=https://www.wmata.com/service/status/details/proposed-august-2020-metrobus-service-changes.cfm |website=www.wmata.com |access-date=31 January 2021}}

On September 5, 2021, service was also increased to operate every 12 minutes daily between 7 a.m. to 9 p.m.{{cite web |title=Improved frequency and changes to Metrobus service begin Sunday, Sept. 5, 2021 |url=https://www.wmata.com/service/status/details/metrobus-service-changes-september-2021.cfm |access-date=16 August 2021}}

As part of WMATA's Better Bus Redesign Network approval, the T18 was renamed into the P40. However, the limited-stop portion between Mount Rainier Terminal and Rhode Island Avenue was eliminated.{{cite web |title=Metro Board of Directors approves Better Bus Network Redesign, new bus routes to start next summer |url=https://www.wmata.com/about/news/Metro-Board-of-Directors-approves-Better-Bus-Network-Redesign-new-bus-routes-to-start-next-summer.cfm |website=WMATA |access-date=23 January 2025 |language=en}}{{cite web |title=Maryland Better Bus Routes |url=https://www.wmata.com/initiatives/plans/Better-Bus/upload/Resource_2025-Route-Profiles_Maryland.pdf}}

On January 10, 2022, a man was stabbed on board a T18 bus along Rhode Island Avenue between 14th Street and Montana Avenue. The suspect ran away from the scene.{{cite web |title=Man stabbed on Metrobus in Northeast DC |url=https://www.wusa9.com/article/news/local/dc/man-stabbed-metrobus-northeast-dc/65-30eac5ab-5138-4b90-a288-98fc15dcecf7 |website=wusa9.com |access-date=12 January 2022 |date=January 11, 2022}}

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