Cash-poor motorists may rest easier this week: The Somerville Traffic and Parking Department now takes plastic. “The addition of credit cards has been a long time coming,’’ Mayor Joseph A. Curtatone said in a prepared statement. Residents and visitors have requested the service, he said. While stressing the importance of parking regulations in the dense city, Matt Dias, traffic and parking director, said the rise in popularity of credit and debit card use prompted the change. Other recent upgrades include electronic parking meters installed at city lots that also accept credit and debit cards. - Matt Byrne