[Purposes] To evaluate the overall effects and heterogeneous mechanisms through which multimodal corridors (where roads and rail transit closely overlap within the same corridor) and their construction sequencing influence traffic operations and urban spatial evolution. [Methods] Three typical multimodal corridors in Guangzhou were selected, and multi-source time-series data were integrated. On the traffic side, a counterfactual approach was used to infer the no-corridor baseline and compare it with observed outcomes, together with analyses of diversion effects on parallel arterials and bus–metro transfer convenience to characterize operational changes along the corridors. On the spatial side, a relative housing price index, production–attraction analysis (P-A), and changes in the points of interest (POI) were employed to assess functional agglomeration and jobs–housing characteristics. [Findings] Construction sequencing has significant impacts on road traffic volume and travel efficiency. Under synchronous construction, traffic volume increases while peak-period efficiency improves. The rail-first/road-later sequence suppresses the growth of road demand, whereas the road-first/rail-later sequence constrains further increases in road traffic after the rail line opens, leading to stabilization. After completion, multimodal corridors generally relieve pressure on parallel arterials, enhance network stability, and improve transfer convenience. Spatially, rail accessibility is the key foundation for relative housing price premiums and cannot be substituted by road improvements alone; commercial services and office-related POIs show strengthened agglomeration along the corridors. P-A results indicate that synchronous construction is more conducive to a relatively balanced jobs–housing pattern, while asynchronous construction more often solidifies a residential orientation or results in one-sided functional concentration. [Conclusions] The traffic and regional spatial effects of multimodal corridors are significantly moderated by construction sequencing. Sequencing should therefore be tailored to local conditions, with coordinated improvements in feeder connections and corridor-side functional planning.