The sign company you used to work for could (probably does) hold the copyrights in the designs you created when working for them. Under the work made for hire concept, the copyrights in the sign designs created while you were an employee, if created within the scope of your employment, become the intellectual property of the company/employer, absent an agreement to the contrary.
If the works are works made for hire, you would not be able to re-create these designs for your own sign business without the express written permission from the other company (and, if you are planning to compete with this business, it is unlikely they will sign over to you the rights in your previous work).
You can have all facts reviewed by an attorney in your area, however, to determine better the copyright status of the designs.